Avery doesn't value the piglet because of its size. It is ironic that the pig will become the hero of the novel and the hero of the Arable and Zuckerman household. Avery highlights a very common and very sad human weakness of only being capable of seeing what is on the surface. Later on, Charlotte will of course play to this as she knows the humans will only be able to read her writing one dimensionally and this, ironically enough, is what saves Wilbur.
Hope this helps! >I love Charlottes Web!<