It would nice to develop our origin story more.

[Hinoto-no-Ryuji]

This is all off course wild guessing from trying to come up with an explanation for how the WOL is able to grow stronger at such a rapid rate, with particularly notable spikes in ability after traveling to new regions.

The basic idea is that Primals may not be the only ones able to benefit from the power of belief.

We don't really need wild guessing to explain our power, is the thing. Our raw strength, while certainly formidable, isn't really that far removed from a variety of other exceptional NPC warriors, and we owe a pretty great deal of our combat prowess to the Echo and the Blessing of Light, both of which we know the source of (Hydaelyn Herself).

While it's not outside the realm of possibility that belief and prayer could empower creatures other than Primals, there's not any evidence of it in the story or lore so far that I can think of, and we'd need that evidence for it to be a viable theory I think.

[Bright-Flower]

We don't really need wild guessing to explain our power, is the thing. Our raw strength, while certainly formidable, isn't really that far removed from a variety of other exceptional NPC warriors, and we owe a pretty great deal of our combat prowess to the Echo and the Blessing of Light, both of which we know the source of (Hydaelyn Herself).

While it's not outside the realm of possibility that belief and prayer could empower creatures other than Primals, there's not any evidence of it in the story or lore so far that I can think of, and we'd need that evidence for it to be a viable theory I think.

It never really bothered me, until SB.

Zenos kicks our butts twice. Which I'm fine with, but then when we get to Ala Mhigo we can suddenly fight him? Why? What made us so much stronger NOW? Is it because we hit level 70? Is level a lore thing now? Is it because we fought in a few more battles? What made them so special compared to everything we went through between 2.0 and Yanzia? Is it just convenience because the story's coming to a climax?

And then Shinryu. Him being summoned was such a big deal Papalymo had to sacrifice himself to buy us time and we turned to Omega to fight it out of desperation, and they fought to a standstill. Then after we fight Zenos, we're able to just...kill it? That easily? Why? Did we just overestimate it before? Did we get zenkai boosts from dbz whenever Zenos beat us up? It just feels jarring to me.

[Jeykama]

Zenos kicks our butts twice. Which I'm fine with, but then when we get to Ala Mhigo we can suddenly fight him? Why? What made us so much stronger NOW?

And then Shinryu. Him being summoned was such a big deal Papalymo had to sacrifice himself to buy us time and we turned to Omega to fight it out of desperation, and they fought to a standstill. Then after we fight Zenos, we're able to just...kill it? That easily? Why? Did we just overestimate it before?

We can only assume that Omega and Shinryu did a good number on each other, weakening Shinryu enough for 1) put it in prison, 2) Zenos to take it over and 3) us to defeat them. Who knows if Omega was impacted in any way considering it's a robot (other than making a crater deep enough to invade the aether sea). I suppose we'll find out more in 4.4.

As for levelling vs. lore, you can never make a 1:1 connection especially in a game without a permanent level cap (such as early AD&D where lv20 means your character was basically a demigod and usually made to retire). Levelling is simply a mechanic where you progress through the unknown of each expansion. Ever tried to fight a lv70 monster as a lv60 job? It's kind of awful because you miss so much and can't really dole out the damage while the monster can easily wreck you. Those quests concerning Wiscar at the Saltery obliquely explain why you're eventually so good at killing monsters - you know their weak points to deal the most damage, something you can teach even a relative greenhorn like him to kill lv70 regional monsters. So experience is a means by which you learn how best to fight certain things as you progress from area to area. Of course this rationale breaks once you switch jobs or use other means to level like PotD/HoH or spamming roulettes etc.
The first time we fight Zenos (who is lv70 this entire time) is lv61, then 64ish, then finally at 70. We've beaten down two more primals (including one that uses a giant sword), explored two new regions, fought a lot of new weird critters, learned some new techniques. This time we don't have to worry about evacuating wounded, Y'sh getting cut in half, or Yugiri being a hothead, it's just us v. Zenos. We probably have a limit break too or can get one midfight. So we beat him - barely. I don't think it's meant to feel like a sudden thing unless you rushed the MSQ.

[Bright-Flower]

I don't really take that 'go for the weak points' thing as a lore explanation for leveling. For one, isn't there also an option you can give to just hit it as hard as you can or something?

Yes we fought a couple more primals, but after all the primals fought in ARR and HW, is two more really going to make THAT much of a difference? Plus when we fight him in Yanxia we've already beaten Susano.

If SW was the whole game, and we fought Zenos at like level 10. But we've been through all of ARR and HW by the time we fight him the first time. And nothing in SB we do really feels like it's that much more extreme than what we went through in ARR and HW to justify that extreme of a power gain. We're not unlocking more crystals with our blessing of light like we are in ARR and then again in HW. There's no lore evicence that fighting primals makes us that much stronger. It just feels to me like 'it's the end of the story, now it's time to fight Zenos.'

[Cilia]

What it comes down to is that whether or not the Warrior of Light's power is tied to the Blessing of Light - or rather, how directly it is - is a highly contentious issue. One the one hand, we know for a fact they're capable of defeating Ascians without it (see Nabriales). On the other, they're smacked around by an exceptionally powerful and skilled Imperial legatus (Zenos) at the start of Stormblood. The popular hypothesis for this discrepancy is that the Blessing isn't granting power unless the Warrior of Light is up against agents of the Dark - primals, Ascians, the Ultima Weapon, etc. This explains how particularly powerful mortals (Resonant Fordola and Zenos) are able to outmatch them, yet Shinryu is capable of being downed despite being designed to rival Bahamut's destructive potential.

... but how much of that is the Warrior of Light's own power and ability, and how much is owed to the Blessing, is still a contentious issue due to them defeating an Ascian sans Blessing.

As for levels... well, remember that you need experience to gain levels. It's not just a measure of how powerful you are, but how skilled you are as well. Zenos isn't (just) too powerful to overcome at the start of Stormblood, he's too skilled as well - remember he was able to block every single one of Lyse's blows with no real effort - and it's not until the Warrior of Light has become much more experienced from their travels through Othard and Gyr Abania they're able to match him. In terms of raw power Zenos probably still outclasses the Warrior of Light (dude is a mountain of muscle - a Roegadyn might be on par with him but Zenos would crush just about anyone else), but their trials and tribulations have honed their skills to the point they can match him even at a power disadvantage.

(If we're going with Dragon Ball references... Remember how Piccolo would have bested Frost in the Champa Arc of Dragon Ball Super despite being at a massive power disadvantage if not for Frost cheating with poison needles? Think along those lines.)

[ObsidianFire]

On the subject of Bahamut vs Shinryu... we don't actually fight Bahamut the way we fight Shinryu. Alissae teleports us into Bahamut's heart and we somehow beat him in there. The nearest parallel I can draw is that it's a bit like how we killed Bismark; instead of attack the primal itself, we attacked what was powering it. Only in Bahamut's case, we killed it from the inside out. Fighting Bahamut like how we fought Shinryu would be like fighting the ginormous Bahamut we see reforming inside of Dalamud...

[Cilia]

Shinryu's card says he was designed to rival or exceed Bahamut's destruction. Not necessarily his power level, but the sheer destruction.

If one remembers their sealings, it's pretty obvious Shinryu wasn't quite as powerful as Bahamut. Bahamut broke out of his seal before it could even finish being applied. Shinryu took some time - at least a few days, since Aymeric made the trip from Ishgard to Gridania on foot, the council was held to decide what to do, and the Warrior of Light & co. were dispatched to Mor Dhona to wake up Omega and wrangled with Grynewaht and his forces before Shinryu broke out of his prison. Admittedly this may be due to his very young age, but... who knows?

Either way, no, taking on full-scale Bahamut would not be a good idea, but there's no reason to believe Shinryu couldn't match him in terms of power despite his smaller size. Furthermore the way Alexander was fought more closely parallels how Bahamut was defeated - going into his crystal core and defeating the core of his essence. (This is likely only possible with the very large primals, with Bismarck beginning to reach the size necessary to have one such core.)

[Jeykama]

I don't really take that 'go for the weak points' thing as a lore explanation for leveling. For one, isn't there also an option you can give to just hit it as hard as you can or something?

Yeah, but just like how you don't like the idea of weak points, I don't think hitting things as hard as possible is the "canon" option :P

I'll leave the whole DBZ power building thing to other people to try to jimmy. Zenos is only a man after all, a very skilled and brutal fighter, but still only a man. It doesn't take us needing overwhelming power to defeat one guy, just more skill and maybe more focus.

[Mixt]

Well, there was that line about Varis sacrificing Zenos upon the laboratory table, even back before the Resonance was ever a thing.

Which makes me think a part of Zenos's strenght comes from being some sort of augmented transhuman.
Ancient Allag messed around with that kind of stuff a lot, and we know the Garleans have a thing for taking their tech and making it their own.

There is that Allagan Drone you talk to to unlock the Warring Triad EX fight simulations, which states that the participant must first consent to "Extensive physical modifications" before participating in the simulation, only to fizzle out/malfunction before it can go any further, upon which the trial unlocks.

[Cilia]

It is implied, but never directly stated anywhere I'm aware of, that Zenos had already augmented himself before he was encountered in-game. However, Elidibus' line about Varis "sacrificing [Zenos] on the laboratory table" comes at the end of Stormblood; it's a translation of his JP dialogue in the stinger. The English line simply has Elidibus call Zenos an "irreplaceable test subject" after criticizing Varis' lack of grief over his passing.

... so yeah, it's relatively certain that Zenos was already augmented even before Resonance, which was developed during the course of Stormblood (implicitly the procedure was completed and perfected sometime between the second bout with Zenos in Yanxia and the return to Eorzea when we were informed of Krile's capture).