You know, I have never seen a bitch have problems having pups because of a size differance between the bitch and the dog. I have seen pups born to a Boxer bitch by a Neo several times, sometimes 60 plus KG differance, a boston Terrier have pups to a Corso, plenty of bull terrier and boxer bitches have pups to Great Danes, English Mastiffs and Bull Mastiffs and a Chi bitch have pups to a large cross bred hunting dog these are the only ones I can think of off the bat where parentage was verifiable by colour or another factor.