our baby is at about 16 pounds right now (5 months). the FB OS is noticeably narrower at the crotch than the fb M. we use fb (M) as a night time dipe and the fb os very rarely ... just not as thrilled with the fit and smaller room for the soaker.<br><br>
Holding the 2 dipes up together, the OS back and fronts are the same width. My fb M (from last year - no hip snap) has the same number of snaps in the lower row, but 1 less snap on each side in the upper row... it will adjust to the same max size for lower row, but about 1" smaller in the upper.<br><br>
FB M fit baby starting at 10-12 pounds, we used prefolds before that. I would go with FB M unless you expect the baby to grow very slowly to 12 pounds. (preemie for example)