Well, there is something to the 'getting fat after spaying'. But, it has more to do with a change in metabolism, activity, and eating too much for what activity she does have. Like a lot of us.

Think of it as if Mother Nature programmed female dogs to have one litter a year and gave her two chances a year to get it done. Along with that went the metabolism to accommodate the stress of raising a litter and recovering from it without putting her life in jeopardy. Automatically.

But Man, in his infinite wisdom, upset that plan with a simple little surgery. Mom Nature, seeing no benefit to informing Man of his oversight, left it to him to figure out for himself. Eventually, it seemed to work out by simply regulating his dog's diet according to her activity and weight gain, or loss. As with any weight loss program, it is inestimably easier to live with by starting early, in anticipation of 'things to come' if nothing is done. One will find out that the correct program is one that varies from week to week, more or less, depending on each dog's life style and demands it makes.