I had a real problem with Bogey and his weight too. It is very hard to free feed when you have more than one pup. You cannot control the amount of food they are eating. When I had my foster Truman who was blind Bogey would eat his food and then help himself to Truman's as well. He put on so much weight that I had to put a stop to it and began putting down their bowls and cut back quite a lot on Bogey's.
Now Bog gets a little less than 1/4 cup twice a day and he is back to being slim again. I will bet that you are feeding too much if you are adding chicken to their kibble. My 3 are all teapots and they get 1/4 cup so if your little ones are on the smaller side they should get even less.
If you stick to putting the set amount in their bowls and picking up the bowls after their meal they will both get the idea to eat while the food is available. It's hard to do but in the long run I can see how much more healthy my lil man is now that I got that extra weight off.
As for weight gain after a spay I have read many times that is just a myth.
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