Vet here. There are several things that cause a dog to gain weight. Even if you feed them and then put their food away, exercise them, and promote healthy playing and sleeping habits, dogs can rapidly gain weight. After fixing a male, or spading a female, a dogs metabolism can get fucked. Even if they don’t eat too much their body won’t use enough of it for energy. Not only will this make them tired and generally make them more lazy, it will mean more food is going to fat rather than being broken down into energy. Female dogs after giving birth will also often gain quite a bit of weight even if they are done growing and continue their usual daily routine and exercise as before the babies. Yes, a lot of people tend to overfeed their dogs and it is very bad for them but please also understand that there are times where it is out of their control and good on them for sticking by their pets side