My girls are around 7.3 to 7.5 kg and would be considered quite small in the UK for the show ring. Tommy is 8.8kg at his right weight, he's a bit skinny at the moment. They all carry weight differently - Tommy carries it as a pad of fat at the very bottom of his ribs so still looks skinny whatever weight he is!
Previously my largest dog was Teddy who was 11.4kg at the right weight, Rupert was 10.4kg. These girls are the smallest we have had.
Generally although they are a toy Spaniel it's not considered a good idea to breed very small - teacup - Cavaliers, they have enough problems without miniaturising them.
The breed standards say 12-18lb or 13-18lb depending where you are in the World! The dogs in the show ring here would be unlikely to fit within that standard now, the bitches just about. This is fashion though and some years back there was a time when they were larger.
I have seen a few very large Cavaliers, the puppy farms do sometimes use Springer stud dogs to give larger litters, at 8 weeks spaniel puppies look fairly similar unless you know what you are looking at - these obviously grow larger than the average Cavalier.
It's tragic to see a Cavalier this large, being overweight is life shortening for a Cavalier due to their heart condition, possibly as much as 2-3 YEARS.
Arrows indicate waist line
This Cavalier is an ideal weight
this dog is still ok but should not be allowed to get any bigger
this dog is very overweight
Photos courtesy of Laura Lang, used with permission
It can sometimes be hard to see how much weight they are carrying if they have a very full coat, it is easier to see when the coat is wet, but you should be able to feel the ribs with just a very thin layer of fat on them, they can carry weight on their breast bone, around the neck and around their bottom - you shouldn't be able to pick up a large amount of fat in these areas.