How much are you feeding your Boston puppy?

My puppy is 15 weeks old and she is looking so thin. I don’t know if it’s just a growth spurt or if I should be feeding more.

She is on Royal Canine small dog puppy formula and I have her on 1 cup a day (1/3 cup 3x daily) based on her expected weight of 20lbs.

I am just curious about what others are doing and if anyone else has noticed their puppies in a growth spurt. Thanks!