I find the sized nappies are a better fit on both of mine because they are both quite long and slim and suit a narrow crotched nappy. Most of the OSFA all all pretty wide, so it was hard at first with Miss M to get a good fit. Plus her legs were pretty much chicken legs, so leg seal was a problem!!!! The best ones I found were bumg's or hautes on her.
I use mainly OSFA on DS though - and he's 2.5 (nearly) so they really do last under they are toilet trained. However, I do stress that he is 13.5 kg and quite slim - he can still fit into most mediums, so I would expect him to fit into OSFA until he's like 5 or something.. lol