The problem is they are athletic first and foremost and malnourished by the astral plane second. Instead they are basically skeletons with skin on the male side.
I'm feeling lazy. First page of "dnd 5e gith" under images on google. Has both official and fanart.
Yeah, they certainly should have muscle on them. I wouldn't even say they're malnourished (since you don't need food on the Astral Plane), just naturally slender and appear emaciated to humans.
Hah, I hoped for obscure gith hairstyle pictures.