Adult weight can be determined by genes that go several generations back. Parents may be small, but that doesn't mean that they can't produce pups larger than themselves.
My boys are full brothers, different litters, and are larger than their parents and much larger than their littermates.
General rule is double the weight at 12 weeks as Verbena indicated. My boys exceeded it big time.
I knew Max would be larger by the length of his paw, but Teddy was a surprise. He was really a late bloomer.