I'm going to let my dog get fat
Why do you want your dog to be fat?
It shows that I do not try to control my dog's activities.
It is nice to give your pet comfort and freedom to an extent.   You are also responsible for providing them with care and keeping them healthy.    Not doing so can lead to more pain for them in the future.
It's my dog, I can do what I want.
If you love and care for your dog, you won't get them fat on purpose. Don't you want to be with your dog as long as possible? A healthy dog with live much longer and you'll get to go on more adventures with them.