The male should be bigger and dominate. The only time I would worry is if he was overly aggressive towards her and can hurt her. If they are happy as clams, he can get as big as he wants. I think the ladies like the bigger males anyways.

Also, you should get a scale. Females should not be breed under 250 grams. Males are fertile at 5-6 weeks of age. Get some pictures, and we should be able to tell you whats up. Are you sure you have both a male and female? There are a lot of factors into why they arent breeding. But, I second the pics.