Bought size 18 months for my 9 month old chunker thinking itd fit him for a while. First time wearing one of the onesies and its a snug fit already! Theyre definitely meant to be slim fitting undershirts. Still great value for the $ and I like that the material  is thin for the summer. Wish i wouldnt have thrown them in the wash first thing as i definitely would have exchanged them for the next size up otherwise.