If its eating and pooping normally (meaning OFTEN since its a chick...lol), then the crop is most likely fine. So, compared to others of the same breed and age, it appears skinny? Or are you only comparing it to the younger, chubbier chicks? And are you certain of the breed? Because some breeds are just skinnier looking than other breeds.
Personally, if it eats, poops, and is active, I wouldn't worry about too much about it. The scrambled eggs suggested is always good, especially for lagging appetites - but that doesn't seem to be an issue here. But they couldn't hurt, since they're full of protein and yummy.