I don't have thin hair but I have fine strands and my twists can look sparse. But I find if I do them small, they look less so. Also if I dunk them after doing them, they fatten and look fuller.
Subsequent washes make them thicken even more:
Also, if you stagger them so that each row is shifted a bit to one side from the one below it so that each twist in the next row lines up with the parts between the twists in the previous row, and so on (as I tried to explain
here), that helps them appear fuller.
ETA Another image that shows how thin twists give the illusion of fullness: