). If you use banana and freeze them they last even longer.
We haven;t seen any images of this dog and cannot tell if weight is appropriate. I have had cavaliers into rescue that weighed close to 40lbs and they were appropriate in weight to their build, despite being well beyond breed standard. Breed standard is a basic guideline for show dogs and cannot be used to assess appropriate weight for any given dog. Any more than saying all women should weigh 125-135 lbs. A vet would be best to make this call for this dog -- someone who can actually see and assess the individual dog in person. 
Murphy weighs in at 11kg, and is super slim. He makes me very jealous
He is very tall and long. As had already been said, with different shapes there is no perfect weight that every cavalier will be. Can you feel or see ribs? If not then a pound or two should come off
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How can you know this though?
My cavalier is basically the smallest cavalier we've ever met. He's about 13 inches at the shoulder, and last he was weighed, he was 14.9 lbs. That's a new top weight for him. He's 3 (DOB 2/7/09). Up until last year, when he started Flyball training, he was right around 13.5 lbs. Once he got into it, he went over 14 lbs, and was generally around 14.5, give or take a few ounces. I've been doing a lot of muscle building work (jumping over and back over a jump, going up on his back legs to touch a target stick, and working on a handstand) to get him more conditioned for when he starts racing, so I think that's why he's a little bit heavier now. He's very muscular for the breed, and has an abdominal tuck, but you have to get your hands under his hair to notice it. I can tell if he's gained a few ounces just by picking him up. Anyway, I am happy with his condition. Most of the vets where I take him are happy with his condition, though one this winter suggested putting a pound on him for the winter. I prefer to keep him on the slightly thin side, basically just making ideal weight. Our vet has multiple doctors, and except that one, everyone is very pleased with his condition. Because he is involved in something that basically involves repetitive jumping, and hitting the box, I think it's more important for him not to carry any extra weight.
And even at 13.5 he is into breed standard weight so I wouldn't worry or consider this to be too small a size. I have two that are 11 and 12 lbs or so (rescues) and they are both in good weight for their build -- no reason to put extra weight on them. Breed height standard is actually 12 inches at the shoulder, so he'd actually be a little tall f 13".
Lots of cavaliers -- at least half of all I ever got into national rescue over 7 years -- were either under or over breed standard weights -- mostly over. So it really is just a guideline for the breed but not a requirement for the health of an individual dog whose build might not match breed standard.
On the health end of things, keeping a cavalier in good weight and even slightly on the lean side is going to mean the dog will have considerably more likelihood of living longer. A study on labradors concluded being overweight cut on average about 2 YEARS off the dog's life and that is in a breed that doesn't have endemic heart disease like the cavalier so keeping dogs fit and in proper weight is so important. Exercise is so important too and doing fun activities like agility and flyball is fantastic! Most dogs just love these activities even if done on the most casual basis.
One real weight concern are breeders who deliberately breed small cavaliers especially anything ridiculous like 'teacup cavaliers' which does have a considerable risk of introducing serious health problems in individual dogs.
