Lord of the Flies is a symbolic novel, and each of the main characters represent aspects of human nature. The first two characters we meet in the story are Ralph and Piggy and, as your questions suggests, they could not be any more different. In addition to their physical differences, these two boys are unlike one another in nearly every other way.
Ralph is symbolic of the physical, which is why he's interested in the physical aspects of being on the island--swimming, blowing on the conch, doing handstands, exploring the island, building huts, keeping the fire going, and more. Piggy, on the other hand, represents the intellect, as do his activities--listing, asking, organizing, planning, thinking. They are not, for now, working particularly well together. Soon, though, these two and Simon will represent the non-hunters and the less savage aspects of human nature.