: are they much smaller than the earlier carrera mkII?

Not at all! The major dimensions are remarkably similar - 3rd gen is circa 1mm wider (the crown is more recessed), about the same less tall and about 1mm higher than the 2nd generation.

Despite the similar dimensions, it gives a very different impression. You have to remember that for the 3rd gen it's all case; no cut out between the lugs and the mineral crystal is near flush with the case. It wears quite differently too, feeling much taller on the wrist thanks to the case (second generation Carrera again has quite a domed plexi accounting for a fair bit of height).

If you're comfortable with the overall size of Carrera 2 and how an automatic Autavia feels on the wrist, you'd be fine with the 3rd gen Carrera. If you've only worn 1st and 2nd gen Carreras though, I'd recommend trying a 3rd gen on to make sure it's comfortable on the wrist before buying.