I find it tedious having to change outfits when I change my body type. Because yeah, the heavy simply looks redicilous in an outfit you've made for the light.

I'd like to have an extra 4 appearance customisations for each character, but I think when I say "extra 4 appearance customisations for each character," you should already see the flaw in that suggestion...

Not totally against it, but it wouldn't be practical from a coding standpoint.

make a new character for each body type - you have 10 of them.

This, ^ +1.