In the first three chapters of Lord of the Flies, what are the differences between Piggy and Ralph other than the physical ones?

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.