Purchased this in size 18 months for my small 2.5 year old (he's 21 lbs).  The sizing seems to be right on going by the weight charts posted by other reviewers.  And the diaper works GREAT!  No leaks whatsoever.  And my son tested it out with a couple of #2's with no issues at all.  He fusses every time that I put it on him, which I'm thinking is probably from the elastic around the legs.  But there weren't any marks around his legs when I took it off of him, so I don't think it's too tight.  He's just picky :)