My Belle is about 18 pounds, she's got a small frame, and we struggle with her plumpiness all the time. Cavaliers tend to be a little obese, unfortunately. My Bobby is around 25 pounds, and when we saw his dad, I literally joked he was a St. Bernard, he was soooo big!!! lol. Bobby is a good size and despite his wonky knee, is really healthy (pray to God it continues).
Bless them, some of the bigger cavaliers do take up a lot of the lap. If mine are positioned on me the wrong way, I actually start to ache (have a touch of arthritis anyway).