Here's a great article from the Whole Dog Journal on overweight dogs, how to tell if yours is and if so, how to help them lose the flab (hint: one way means owners also start to lose... more, longer walks! )
Firs off, the article notes that, yep, our cavaliers are one of the handful of breeds with a genetic predisposition towards gaining weight, so we all have to be very aware of feeding amounts, exercise, and the size/weight of our dog (weigh regularly and check shape).
There's lots worth reading in this, including why neutering a dog is NOT a cause of weight gain (overfeeding when they now need 25% fewer calories, is -- something we all can easily control )
There's a handy drawing too for deciding if your cavalier or other dogs are fit... or fatThe bottom line here is that spayed or neutered animals will not get fat just because they lack their gonads. True, they will likely need fewer calories, after the surgery – but so long as they are fed foods that provide them with some caloric reduction, and so long as they continue to exercise adequately, they will not gain weight.
From the left, they are fit, fat, obese...!