Diaper question urgent! Seventh generation do they run big or small?

Do the size one diapers run big or small or true to weight? Would Size 1 work for a sort of skinny baby 18.5 lbs. The weight says 8-14 lbs, but I really don't want to go back to the store.

Longer version: cloth diapers don't seem to be able to hold enough pee now that my daughter is 8 months. We were using bambo, but ran out so my husband picked up some seventh generation diapers at the store and got the wrong size. Will a size 1 work for an 18.5 lb baby or should I return them?