Luke is probably the only lean cavalier my vets have ever seen, and they do usually back off when I mention the activities he is in.
Heh! Yes I think that IS an issue... also I think vets often fear saying anything about weight to owners. Eevery time I go to the vet most of the dogs in the waiting room ar fat and regularly there are incredibly obese dogs too, often with health issues they might not have if they were fitter (joint problems, heart problems, diabetes etc).

My vet also says my four are just about the only cavaliers he ever sees that are an appropriate weight . Sadly, however, I have willpower for the dogs, and less for my own waistline...