How to control my dog's weight

Hello, everyone! How are you? Are you doing your best to study for exam?
Well, do you have a dog? or other pet? I have one, it's a collie dog.
She is now 10 years old, already grandma. And my family and I have to control her weight.
However, there is a big problem. My family is living with my mother's parents and great grandma. And my grandpa is giving us a favor to take her walk everyday. Then, he gave her a piece of cream bun or spread bread with jam. It is too much calories a day to manage her weight. It is the same problem for human. Having too much sugar would develop diabetes to even dogs. Therefore, I explained what my veterniarian said and warned my grandpa not to feed her such sweets too much and take out jam or cream before gave it to her. However, my grandpa just ignored my warning and kept giving her sweet behind me. I felt strange because no matter what we do like setting a limit to have a meal or walking, she just didn't loose her over-weight. Hah...
How should I watch my dog's weight and my grandpa?? I should need to stop it for my dog, but it is little hard to say to my grandpa because he is helping her to walk everyday too. Well, I can ask my mother and sister to warn him, maybe.