I know it's been mostly covered, but I'm just going to put it out there that even within humans, in our world, you have people who are both deceptively strong and deceptively weak, for their apparent size and build - it's not as much variance as you find in completely different species, but even among you humans you find folks with differing muscle densities, enough to make a visible and unexpected difference. It's no big reach to imagine that gith have above-human-average muscle density, in the long run.
(The "lithe race with unusual strength" goes back at least as far as tolkien elves, but I'm sure it goes back further than that too ^.^)
