jam: yah, get the OPKs from the business advertiser here on this forum. she's quite responsible and friendly.
catz: LOL resident hoarder ... i hoard all, no more left! hahahaha, just kiddin!
ruggles: eh, diapers don work that way lah. if S is too big now, there's no way an M will fit him.
try putting him on the NB size? it may stay better.
the weight on the packaging is just a rough guide lah, it really depends on your kids' size.
another factor is the cutting, some brands have generous cutting eg nepia.
eg my son has a big butt, so he's already into XL diapers even tho his weight doesn't correspond with what's recommended!