Why is our dog overweight?

She’s a 3 year old bulldog. She’s supposed to be big, her dad is around 85lbs and mom 70lbs. We took her to the vet and they told us that she’s 95lbs and supposed to lose 15 pounds. She doesn’t look overweight to us. She eats 1+1/2 cups in the morning and night. It’s a weight restriction food. Gets walked plenty. How do we get her to lose weight?