absence of gravity or lower gravity on space causes osteoclasis and loss of muscles.
other races should thank newton for discovery of gravity

There is no need for big muscles with the moon-like gravity of the astral plane.

In the original Fiend Folio, they looked even skinnier..more like skeletons and had no nose, only a hole in the face like a skull. They have been retconned since then to appear like a cross between a frog and a monkey with a more human like appearance.

Same reason why the dwarves, gnomes and halflings are all shorter; Biology. The potential range of body mass of the two Gith species doesn't go that far. Their muscles are more efficient though, so they can still be as strong.

Githyanki (and-zerai) evolved in the Astral, and strength does not really matter there for most purposes, strength checks are replaced with intelligence checks. The way in which the Astral works in BG3 is fun, I like it, and it is also a little bit simplified. And yeah, a gith with 20 str feels kinda weird... well, as weird as a gnome or a halfling with strength that high. But not allowing it would be an arbitrary limitation, and that would be unneccessarily annoying for players.

Anyway, maybe just make up some sort of headcanon to explain how that abstract stat translates to the imagined reality of your character ;-)

bulk isn't directly related to strength... look at the champion weightlifters ... they are (mostly) not built like arnold / conan days but many of them are not any more bulked than an average joe yet they can heft 500+ pounds easily. Bulk is from doing a regimen that is designed just for that (to look good) and while yea, those guys are strong too, they are not strong the same way and their goal wasn't smashing power but cosmetic result.

Originally posted by jonnin:

bulk isn't directly related to strength... look at the champion weightlifters ... they are (mostly) not built like arnold / conan days but many of them are not any more bulked than an average joe yet they can heft 500+ pounds easily. Bulk is from doing a regimen that is designed just for that (to look good) and while yea, those guys are strong too, they are not strong the same way and their goal wasn't smashing power but cosmetic result.

If you look at a monkey they don't looked jacked, but they are about 2x stronger than you as their muscle fibers are a lot more dense. Gith are kinda like that.

Ok but what about orks ? Draco men ? Why can't they be super muscular too ?

Originally posted by Jack:

Ok but what about orks ? Draco men ? Why can't they be super muscular too ?

Because they both already are by default.