XIX | X | I

for example, caius and jiub. their stance is a little different too, given the difference in waist-width. mer just have a slightly more alien, angular look, almost insectoid with the ribbing on the muscles. as a contrast here's menelras, a bosmer slave from tel aruhn:

notice his muscles are smoother, not as ridged. i couldn't find a good shirtless image of an altmer, but here's an orc, lorzub gro-bogla from caldera:

thankfully there is no shortage of shirtless nords to choose from so here's the first i found: abbard the wild outside bal fell. tbh they look about the same as imperials like caius - maybe a little longer, which almost makes them proportionate to the bretons moreso than imperials.

and finally for the non-beast races, let's look at redguards. this is jon hawker, a redguard from the dungeon nammu, is is actually an avatar of zenithar himself. looks about the same as a breton to me.

khajiit and argonians seem to both use a slightly modified version of the dunmer bodies. this is j'saddha from the daedric ruin ashalmawia, and big head from the sheogorath quest up in sheogorad.

You're right, the actual chest mesh is different between male men and mer. In Morrowind, every body part is a separate model - chest, groin, upper legs, knees etc. and there is a real difference in the mesh shape.

For the differences within categories, races have different height and weight settings which are fairly simple - higher weight makes the character stretched wider and height taller.

Confirmed, no difference in the female chest mesh (above) compared to the male chest (below)