i buy size 5 for my daughter (13 months) though she is not even 20 pounds because i'm not about the "perfect fit." she needs some back coverage... i remember those times in the first 3 months when there would be no poop in the diaper, it would be all up her back... the force is strong with them.
she was 7.7 at birth, jumped to 9 after the first week and a half, and has steadily gained about a pound to a pound and a half each month...
it is amazing how they look at you, study your face...