In the eso character designer it has the exact same interface with the strength/weight/skinny triangle and it functions as you'd think. The further you get from strength the flabbier you get.

In fallout 4 at maximum fatness your biceps are bigger than at maximum strength, you get giant thick thighs and a barrel chest that somehow has 6 pack abs.

How does bethesda continually mess up things they have already done properly in the past?

Because FO:4 is full of "good enough" elements. During the character creation development someone would have said "We need the body triangle! Can we just scale the muscles up? Good enough."

Ive been trying to make a bigger type wanderer lol, always looks slightly off. I try to model it after the guy in that vault.

Another reason would be that While Bethesda Softworks/Zenimax is the publisher, the team that made Fallout 4 (Bethesda Game Studios) didn't actually make ESO, Zenimax Online Studios did.