Duke is a big boy, he was 9lbs 4 months ago, but my parents over fed him while I was away and he is now 15lbs

we are slowly trying to get his weight back down because the vet classified him as overweight.
But I never thought he was going to be as big as he turned out, his mom is petit, I didn't see his daddy but I know he was bigger than the mom.
I was told that Lucie should be around 5-6 lbs fully grown. I took her for her first vet appointment at 11 weeks and she was just under 2lbs, she was supposed to go into today but they called me and said they had an emergency and needed to reschedule so she's going in Monday a day before she is officially 17 weeks, and we'll see how much she has gained.