
I think it depends on breed, and very much on what background information you have about the lines.
I have a very small breed, known for bad whelpings and C-sections. I have been working for years, towards a self whelping line.
A few years ago I kept a male pup from one of my litters, hoping to show him. I had used a nice dog from a good show kennel. My boy (like Topsy) just grew and grew! Yet the sire weighed 4lbs.
I was disappointed, but once I keep a pup, if show potential doesn't pan out, they still stay. This boy had super type and coat, but I wasn't prepared to mate him to a small bitch. One of my girls is way over size, and I felt that I had nothing to lose by a test mating.
The litter, produced a year ago, was 3 normal size pups, self whelped. One girl grew very big, and went to a pet home. The boy was nice, but I didn't want another male, and he went to a friend. The third pup, a bitch, was chosen by my mentor as the pick of the bunch, and I kept her. She has been a good bet! Definitely within the show standard which is 4-6lbs, and I'm delighted.
Although I admit that, even knowing the size of dogs in his pedigree, I wouldn't have had the nerve to put him to a smaller bitch.