How old is your little one and what size is he/she wearing?
Elliott is 9 wks today and is mostly in 3M clothes. He has a couple 0-3m that still fit, but mostly those are too short. I just put him in a couple 3-6m onesies that fit better. It seems like the clothes with a range (i.e. 0-3 or 3-6, etc) are smaller than the ones labeled with just a single month (i.e. 3M or 6M). You notice that?
I think he will be solidly into 3-6m clothes in a couple weeks. He isn't super chubby, but he's really long so things that snap at the crotch or that have feet tend to be outgrown much faster.