At the beginning Piggy was confronting everyone how they should have built forts and shelter first besides worrying about the fire. He says, "The first thing we ought to have made was shelters down there by the beach. It wasn't half cold down there in the night. But the first time Ralph says 'fire' you goes howling and screaming up this here mountain.
Another reason why Piggy would make a great leader is because of his knowledge. He told Ralph that the conch would be the way to get everyone's attention and gather them for a meeting. When Ralph asks Piggy how his friend blew the conch Piggy explained and told Ralph, "You try Ralph. You'll call the others."
Go on and try it Ralph!
Another reason why I piggy makes a good leader is his intelligence because he suggested the group to make sundials to keep track of time. And with this intelligence he isn't cocky either. He says "We got a lot of sticks . We could have a sundial each. Then we should know what the time was."
I've got an idea!
Now people may disagree with me because people may focus on Piggy's physical health with him being bigger and him having asthma. But honestly he is still able to go with Ralph and Jack on adventures so in the end his health doesn't matter as long as he still has his kindness and intelligence. And plus the only reason for his size is because he can't exercise because of his asthma.
So yes I think Piggy would make a great leader because of his kindness, Intelligence, and his care for others. He is clever and would be find leading the group.