Tank health is not the be-all end-all metric of gear.
I run Experts (and Alex) in Str right side. So my health is ~14K. I've yet to find any occasion where Fending right side (which puts me at ~18K HP as a PLD) is required. Health does not = Mitigation. We get no additional DEF stats on fending gear. Sure its a small buffer at the start of a pull (assuming full health) but it wont mitigate anything. The whole HP issue has been hashed over time and time again on these forums.
It sounds more like you encountered a tank who believes Cooldowns are something you use when you hit 20% health or a RWAR I TANKZ IN DPS STANCE tank. Sadly I see alot of tanks who do this. Hit low health thresholds and then start poping every CD they have. If your that low in health its abit late to be poping CDs (Excluding Hallowed / Holm / Living Dead) I see alot of tanks who are in the mindset that CDs are something you use only when your in trouble, and they try to speed run. Thats the issue, not their HP pool. I see alot of tanks who think that pulling 3 packs in Del / Sword Oath / No grit is a good idea with zero CDs used. Neither of these are HP pool issues, they are stupid tank issues.
Like I said above, I run 14K HP, I use CDs and if I'm pulling alot of MoBs I'll be in the stance to deal with it (Some packs you can pull in DPS stance with a CD or two) I've yet to encounter an Expert Dungeon boss I cant tank in Sword Oath, when using CDs. Having 20K Hps does not a great tank make. Using CDs correctly, using stances correctly, positioning etc .. make a good tank. HPs is just gear ...
You mention that the tank was having aggro issues, So i'm going with - Str Right side, DPS stance, No cooldown usage. The trifecta of bad tanking.