diaper size?

Allie • Lost but not forgotten 03/08/16 Blessed with a beautiful baby girl🌸 Team Save The World 🌎 ♻️
My baby will be 2 months on March 31st and she is barely fitting her newborn diapers anymore. The size 1 seem just a little big though but fit better than the newborns. Just curious but does size one mean 1 month or 1 year? Lol