When Piggy emerges from the Jungle, he is described as "very fat" and the way that he walks clearly indicates that he is not exactly athletic but awkward and ungainly. Piggy also is really the first to point out, audibly at least, that there are no grown ups. He asks the question and Ralph answers with the confirmation that there are no grown ups.
He also lets on that he has asthma, that he is not allowed to run and it is obvious that he has issues with his vision given that he has thick spectacles. The reader could also infer that he is delicate, at least when it comes to his digestion as the fruit he ate has given him the runs. Later in the chapter he is told that he cannot come exploring to the top of the mountain; clearly he is unfit and not just because of his "asthma."
Perhaps most importantly, the reader also soon learns about his nickname. He tells Ralph in confidence that he was called "Piggy" by the other boys at school and that he feels quite self-conscious about it. He is trusting enough to tell Ralph and quite hurt when Ralph tells everyone else about the nickname.