Those things snap down really small! I haven't tried it on a newbie yet but I can see it fitting easily, I'm expecting a 8-9lb babe though. Moose can wear them easily, so they'll get used even if they don't fit Muffin at first.
My baby fit into the one-size dipes right away, SOS and Wonderoo. We didn't have a Mutt yet when she was born, but she would have fit that too, easily.
The one-size dipes were my favorite when she was small. She grew faster than I expected, and suddenly most of the diapers didn't fit her. The one-size diapers fit throughout, and I wish I had purchased more of them while pregnant.