Buying a Leather Jacket and Being on the Slim-Skinny Side

GQ's guide on how to buy a leather jacket.

I read through the guide just after buying a leather jacket for myself. In spite of that, I'm very satisfied with my purchase and I wouldn't have bought it if I didn't think it's a good buy.

Realization: it's hard to find slim-fit jackets and buttoned shirts for me. Size S is still a bit large for me and not all brands offer XS sizes.

So I'm wondering which one is more likely to happen: overwhelming majority of designers offering size XS (or adjusting size S to fit people of my type) or my buffing up? It may be a tough choice considering that the American market has, I suspect, only a very small segment of size XS people and that I always find what to me are more interesting things to do than working out. Running, basketball, and very occasional push-ups are my only exercise.