When I first bought my medium onesie from baby-pants I noticed that the snap crotch area was way to low and really didn't hold up my diaper all that match. It felt more like a large. I tried the washing machine and that made it shrink a little. I remember I read in a thread somewhere (maybe here) that you should boil water and place the onesie in the boiling water and let it boil for a bit.
I did this twice this week and my onesie has shrank significantly to the point where it is a little tight without wearing diapers. So if your onesie is a little to big on you, you should try this technique.