I ordered the size 12-24 month heartbreaker onesie with pants for my 1 year old son who at the time weighed 22.5 lbs. & was 33.5 inches in height. The onesie fit perfectly to his body, you could say it was form fitting and quite snug. He would wear a size 12 month (which fits tightly) or a size 18 month (which fits loosely) in Carter's.
This was a great purchase to celebrate my little one's Valentine's Day Birthday. I've washed and dryed it in the washer/dryer machine 3 times now and the lettering on it has stayed in tact, though since most if the material is cotton it has shrunk a tad bit as to be expected. However, it can still fit my son though it may be nice and snug.