If they are only 6 months of age now and they've been together about 5 months then you put a male and female together when they were only about 4 weeks of age? Be glad they didn't breed back then as that's way too small for a female rat to carry a litter successfully. It's better for your female rats to be raised apart from the male until she's about 4 months of age and around 250 grams of weight.

It's perfectly normal that the male rat will outgrow the female in size. Most every male rat at maturity is bigger than most every adult female rat (not always, but most of the time).