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 )

The 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.
There's a handy drawing too for deciding if your cavalier or other dogs are fit... or fat

From the left, they are fit, fat, obese...!