When Evie was a baby, she spent the majority of her time in a sz 1. She wore them from birth to 6 months, when he thunder thighs outgrew the elastics. After a short run in sz 2s, she started to slim out, burning off her calories learning to walk, and around 11 months we moved her back down. She then wore those sz 2s until she was almost 2 years old.
William on the other hand, moved out of his sz 1s around 8 months, due to his thighs as well! But even though he started walking soon after that, his thighs have not changed. He has been pretty consistently on the first set of snaps, but I don't think he would fit in one comfortably.
This is why it is so difficult to guess when a child may move up in AppleCheeks sizes. Age, weight, or even size can vary so much between children, that one cannot stand as a guide for others. Though the diapers come with suggested weights, 7-20 and 16-35 lbs, you notice they overlap significantly. I am sure this is because those wise ladies know that there is no way to guess just how diapers will fit on any particular child.
I always recommend that, when in doubt, buy both sizes to try! If size 2 is too large, then put it aside for the future, there is a very good chance they will grow into it, sooner rather than later. If the sz 1 is too small, then you have a perfect baby shower gift for a friend, so you can spread the AppleCheeks love to a new Mama.