My vote would be to do with the size of book and the little hands that are going to hold it. Will it be available in paperback to start or only hard cover? How long in hard cover until available in soft cover? I agree that throwing HP in a backpack was difficult. Also for sneaking through at school the smaller books passed easier than did the bigger/thicker books. (I use to sneak books into my kiddos class in her backpack to read after she had finished the work or it was individual reading time.)