[gemy]

I agree with all the other posters: 1) take to the vet first - get their assessment ruling out any health issues that might create this weight gain - including thyroid disease.

Then exercise and measured food control are your most important defense against obesity in dogs. BTW it is quite common to see weight gain after neutering. Not necessarily immediately, but it kind of sneaks up on you. 6mths to one yr later, your dog is up 1/2lb or even 1lb.