sizing puzzle

i noticed that the sizes on bb's clothes arent exactly very accurate. how can my boy be wearing rompers meant for 12 mth old babies, and some of the tops indicate its for 18 mths n ethan can wear them too! i mean, i know he's on the chubby side, but still?!?

case in point: the romper ethan's got on in this photo is for 12 mth old bb. doesnt seem so rite?

ps: thks again aunty LY for getting this romper for ethan - fits him real well! ;)