Why?

running from dragons and the occasional deranged dovakin is great exercise

Because they didnt implement verlet surface at the game beginning ... Check the fat giant on game jam .
Apart that the system they implemented doesnt allows belly deformations and body deformations that is beyond musculature... Some other games allow to make a balancement beteen body fat/size and musculature....

Probably you will get that in a future bethesda game , but in skyrim unless someone dedicates to make specific shapes for each npcs in a mod , is not possible.

Fat armour?