Ethan peck

I swear they must have purposely given him a uniform that's a size too small. He just really fills it out.
While they do make the uniforms enjoyably tight (and I'm sure a good part of that ass is Ethan's), the men's uniforms (possibly the women's too) are padded to create a more uniform (no pun intended) silhouette that also hides underwear lines (and bras on women) and other body imperfections. Jason Isaacs, Anson Mount and Doug Jones (who doesn't wear padding to increase Saru's alien look) had talked about this fact on their social media and at conventions. Mount made a joke at STLV last year about being afraid to fart in his uniform because he's worried at the padding is flammable lol.