My dog is not fat. He's perfect! In fact when I got him (he's a rescue), I thought he might be a little too thin, but the vet said he was perfect and to keep him there. So that's what I try to do.
However, my dog is a compulsive overeater. Also, HE thinks he's starving and might never be fed again, even though I have had him for five years of two meals a day plus some treats. He is a hungry puppy! So I can't believe him when he cries for more, because he does that when he has just gotten into the kitchen and eaten a bag of dog food and a box of cereal.
Lately, I've been exercising more. And he's my workout buddy, of course. So since he is not on a diet, when we do a lot of walking or running he gets a little extra food. That's been my system for years, and it's seemed to work. But I used to live near his vet, and I'd stop and weigh him once in a while and adjust accordingly. Now I don't, and I have trouble weighing him at home because I have to hold him in my arms on the scale and he squirms and it gives me an error message. And he's acting very hungry (but not really more than usual), and he is pooping less, and his poops are smaller. I'd run him by the vet here to weigh him, but I'd have to take time off work to do it, so... How else can you tell if you're starving your dog? I don't want him to suffer for my fitness.