The really baby ones that come in 6-12/12-18 months are MASSIVE! DD had to wait over a year to wear the ones I bought when she was a few months old, by which time she was walking and needed ones with soles.
We've had the Boo style too, which are also sized in months, and pretty big.
Not sure about the sizing one they get to proper sizes though.
We go 6-12 mth ones and they were pants when ds was 4mths. They wouldn't stay on at all. But I love mine to death. I may have to get some for dd (9). Although I suspect she may object as she has given up leather (sigh)