
If they are both expereinced then placing the bitch on a platform might help, but in my experience a dog should be able to manage a small bitch of his breed, even if he is on the top size.
My bitch of 19inches had no trouble being mated by a 21 1/2 inch male.
Maybe the bitch is not hot to trot yet, and he can't be bothered to make the extra effort until she is :rolleyes:
A male I bred who is a good size had a small bitch as his first with no problems and both were maidens,(though he had tried to mate a difficult bitch previously).