As a general point leading out of this discussion (and not directly related to the question!), it is really, really important to stress (and for people who care about this breed to understand!) that there is no such thing is a 'miniature cavalier' or 'teacup cavalier' and breeding undersized cavaliers deliberately directly impacts their health -- and NOT in a good way!! Miniaturising is known already to carry some health risks for some breeds and deliberately breeding extra small cavaliers -- a sure sign of the worst possible type of trash breeder -- is an absolutely appalling activity and ranks up there in my mind with knowingly breeding dogs with heart murmurs, syringomyelia, dry eye or any other affliction that involves pain and suffering int his breed.
There are very often health reasons why dogs are very undersized, ranging from serious disease to malformed systems to neurological problems to being the run of the litter -- and for those trash breeders selecting for smallness it is inevitable that they are selecting for POOR HEALTH as well. Of course they DO NOT CARE -- they only want to charge more for their 'teacup' dogs and won;t have been doing anything to safeguard health in the first place.
Cavaliers really should not come in all shapes and sizes at all except within a limited scale -- there is a breed standard, and the vast majority of cavaliers should fall within those weight limits or around them (more often they are larger which almost certainly has fewer possible health problems than undersized dogs). A few will naturally fall below them too of course, but this is less usual and certainly should not be a goal of breeding. In a couple of years of doing breed rescue I have had only ONE cavalier that I have directly dealt with fall below the breed standard (many above, but usually not more than a couple of pounds at most). Just NB I am NOT saying all undersized cavaliers have more health issues; I am saying that all else being equal, undersized dogs are far more likely to be small
because of a visible or hidden health issue and there are direct knock on problems related to breeding undersized dogs; whereas this is generally not true of larger, oversized dogs.
A female cavalier that is 10lbs at 7 months is possibly undersized or maybe just on the low end of breed standard or slow growing... and while that isn't necessarily an issue at all (and she will still add a little more weight and I'd guess will come in at the bottom of the breed standard, maybe 13 lbs) if siblings are considerably larger I'd definitely want to talk to my vet just to make sure there are no problems. Litters generally tend to end up pretty close to the same size (though there are always exceptions) so it would just make me a little concerned, enough to want to follow a dog's progress and talk to a vet. If she was meant to be a show dog, however, there will be issues -- her small size now will likely mean you have a dog too small to be shown or perhaps that will be seen as really not solid enough, and that probably should not be bred if she stays way down in size. If I bought such a dog as a show prospect I'd be back talking to the breeder about the situation. I think most show breeders would acknowledge very small cavaliers downat the bottom of thebreed standard don't tend to show well and are not seen as solid enough.
If you live in a warmer climate that may well affect coat but a 7 month old generally doesn't have much coat anyway.
