Here are my pups in their Puppia's. This pic was taken last March so it's a little out dated but it's better than trying to find 5 different pics of my pups wearing them individually. All still wear the same ones for their walks & all are the over the head kind.
Milo weighs 10lbs (yeah, I know!) & his is a size S. It fits fine still but I can't let it out anymore. Not that I plan on letting him get any chunkier.
Matilda weighs 8lbs & hers is a size S & fits great.
Maxwells weighs 6lbs 3oz & he wears a S. If it wasn't for his big head he could still wear the XS but I don't think they make the heads big enough for the S.
Maribelle - I don't remember what she weighed in this pic (I'm guessing 4lbs) but she wears the same one now & is 5lbs 14.5oz. Hers is an XS...her head isn't as big as Maxies.
Little Marley - we don't have a pic of him in one. He doesn't yet have his own but he will be getting a sky blue in a style like the others...in a size S since he has a HUGE noggin. hehe Right now he's just using an older black XS one of Maxie's.
I found that once my pups hit the 3lb mark they could fit into the XS's w/o them constantly coming out of them when walking. The step ins DO run a bit smaller...I don't remember them having an XS in the step ins but if they do - I think they'd be great for the tinier pups! None of mine mind the over the head thing. Marley is still adjusting to getting it off but I just flip the neck part of the harness inside out (after getting his legs out) & let him slowly back out & it seems to be working great.