They might not look like big versions of them but the beast tribe behind Titan is living underground and live their life with rocks and the treasure that the underground gives them. Titan being a big rock like primal makes sense in that way. Also everything on him screams protector. Like the rock that you can hide behind and that will protect you from the bad things. Ifrit looks quite reptilian thus fitting them quite good. They are fierce warriors thus his fire, horns and behavior also makes sense. Heck I do like the behavior of the female ones too. Garuda being so cruel, Sophia being about balance even if it means death and Lakshimi trying to give a good live by living in ignorance and pure bliss. I am just sad that they look so much more different then the male primals. Honestly Ramuh is the only one that looks human. I know that they might be that way because of some legends but over such a long time its kinda strange that they did not change to a more beastly version. (Sophia was caged so she gets a pass on that) I mean I dont even want them to be completely changed to something beastly like the male versions (even though that would be awesome imo) but at least some more parts that make them less human. Like giving Garuda a bird beak and maybe more feathers. (At least she is the more beastly version of them all x)) Lakshimi could easily look more like their tribe if she had way more scales, slitted eyes and maybe showing a bit of her existing tail. She would still be quite beautiful but more believable (for me at least).
But yes in the end all it comes down to is fanservice. Just the decision alone to have them be remembered that way is already fanservice since this gives them the excuse to show them that way. I just find that sad but sadly cant change much about it. (Still like their character and the music though)