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......Tommy Tuppence 2000-2012 and a half share of Faith.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.