The fact that it doesn’t make Thematic sense is irrelevant.
The game is about numbers in the end. That’s why they pop out when you shoot things.
Interesting gameplay design boils down to mechanics.
It makes mechanical sense to correlate IB armour with Moze max health. It’s one of the things that ties them together, other than a cute storyline.
It gives a framework for her other skills to be built from, and a place for the other skill trees to spring from.
I’d go as far as to suggest, from a design perspective, it was Moze’s core concept.
The skill tree themes of shield, mags and splash were likely developed later to add complexity.
The sheer fact is you can make Iron Bear indestructible, using the right combination of skills and gear. Whilst piloting Iron Bear, Moze cannot die, or even be put in FFYL.
All things being equal, before even talking about damage, that’s a pretty powerful thing, considering the duration that can be obtained.
So that fact needs to be balanced.
In this case, it’s Moze on foot. You want massive shields? Sacrifice massive armour. Mag size? No reload skill. Damage? Survivability.
Ugh. I wanna delete this because I feel more than a bit preachy. I don’t mean to be. I love discussion about balance and I’m into the numbers side.
TL;DR sometimes things won’t make sense canonically because math and mechanic nerds like myself and others will use any bit of wonky math to break the game and make innocent things overpowered.
That’s the real balancing act for Gearbox. Making a game that’s entirely math based scale from casually shoot things to absolute challenge.