When did you start using BG one-sized diapers?

Babies have been using BG newborns up til now. I think it might be time to move up to OS diapers since DS is peeing out of his BG newborn diapers with prefold insert.

I have mostly BG 4.0 and Flips. I have some BG freetimes and elementals for night time use.

Will they fit my 8 & 9 lb babies? (For some reason, I had 13 lbs stuck in my head for OS diapers, but I'm seeing now it says 7 lbs.) Will the diapers be pretty big on them since they'll probably be snapped down all the way?