I know this is a common concern, but still I hate these questions as it implies cavaliers should be a certain weight. As others note, there's a very large variation even within the breed standard of 12-18 lbs. The tendency in the show ring has been for many cavaliers to either hit or exceed the upper limit for a while now, especially males. Most healthy males are larger than breed standard, amongst UK show dogs and US show dogs, going by breeder discussions, and some have raised whether the breed standard should be changed, especially as most breeders feel cavaliers at the bottom weight are not great breed examples, too small framed without enough bone or good build -- unlike say chihuahuas or papillons this isn't a dainty breed as the breed standard makes pretty clear. I've seen concern that some are breeding such small slight dogs that B&Ts look more like short-faced longhaired dachshunds. You'd really see very few showring cavaliers under about 16lbs, male or female.
Many researchers currently believe it is better for the breed to be larger rather than smaller, and that something in a dog being smaller probably means a greater inclination towards SM because the condition only occurs in miniaturised breeds. Some trigger is thus set off genetically, most likely, by breeding a downsized dog.
Unfortunately there are scores of breeders who aim for extra small undersized dogs as if this is a selling point (and even charge extra for their unhealthy approach to breeding) -- it never should be as some natural cases aside, smaller dogs tend to be extra small for health reasons or breeding choices that a good health-focused breeder would not make. A breeder well-informed on SM and research findings will be unlikely to have small size as a positive. At the same time there's no direct correlation in individual dogs, and both larger and smaller cavaliers have SM -- and most cavaliers will probably eventually have SM as even the main vet for the UK Kennel Club, himself a geneticist, now seems to recognise (asked about incidence in cavaliers, he noted at a recent breeder seminar that reports he is getting indicate now that in cavaliers 60% or more will end up with SM). But researchers generally have noted that smallness brings consequences in bone development -- every choice brings unintended conequences many poorly understood -- and seems potentially linked to selecting for increased risk of SM -- as does a very short muzzle.
A vet can help determine if an individual dog is within a healthy weight -- what other people's cavaliers weigh is pretty irelevent.
As noted, an individual dog should have a clear waist and its ribs should be easily felt but covered with a smooth layer of skin and some fat (eg not bony). Breeder Laura Lang's page on correct body shape and what to feed is very good on this issue.
In rescue over the past many years I've had cavaliers as small as Penny above
, and as large as 40+pounds, all in good weight for their build. Penny actually was pretty underweight when she came in and is still filling out and of course is still young.