why wont my puppies wont gain weight?

TL DR pointers: Yes they have been wormed and up to date with all shots They are about 3months old Husky Boxer mixes (one male one female) I feed them 2cups a day each of half Taste of the Wild puppy mix, and half filler (1part kibbles n bits 1 part beneful mixed in a air tight tub to save alittle money) I also give them fatty treats like freeze dried chicken aswell as turkey necks and sometimes some beef cubes when I can afford it on top of their dinner.

Theyre eating more than my fully grown pit bull at this point who is a chubby fuck at about 1.5cups a day. im afraid if I feed them any more quantity theyre going to pop! I took them both to the vet after their last growth spurt (two weeks ago) and said they were slim. he agreed they were doing fine before at 1cup a day then after the growth spurt they got amped up to 2per day because of how suddenly they lost weight so the vet did all kinds of tests, feces blood etc, everything seems find all we can think of is that their metabolism is crazy high, any tips?