I'm going to call out my grandma for overfeeding the family pet.
Why would you want to treat your grandma that way?
She was just being horrible and I don't want the pets to suffer from obesity.
You're right that people shouldn't overfeed animals, or give them any amount of food without the owners' permission.  However, it is wrong to call out your family members in this fashion.  Try to ask them politely not to feed the pets, and give them your reasons.
Nah, I'm going to tell my grandma how she needs to start listening to my wishes in regards to the pets.
Do it gently and try not to hurt her feelings. Grandmas are always keen to feed their loves ones. I am overweight because of my grandma and I love her for that.