The UK breed standard states a weight range between 12lbs-18lbs but there are many adult male show cavaliers that are over 18lbs.
I think that breeders have been quite successful in bringing down the size since I showed my Champion boy Monty (see my avatar) who was a big dog weighing 22-23lbs, but there are many cavaliers bred by exhibitors that are much too big or too small for the show ring.
The boys I have here are sizeable. Tommy weighs in at 28lbs ( poor boy is on a permanent diet because his weight is not helped by the gabapentin) but as far as I'm concerned that is all the more to cuddle.
Cavaliers......Faith, The Ginger Tank and Woody.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.
Remembered with love......... Tommy Tuppence and Fonzi