I bought the large size for my 20 month old and even though it says for 22-25 lbs, he is 30 lbs and the bottoms still fit him well. The shirt, however, was a little snug but he carries his weight in his tummy:)
My only complaint is that the bottoms are so extra absorbent that they take forever to dry, even in the full sun. You'd do best to have a backup swim diaper or some disposable swim diapers on hand.
Reviewed by: Lauren from Alexandria, VA.