I don't think it has anything to do with the weight of your animals at all. I've had 6-7 week old males (90-100g) breed with larger 200g females double their size with no problem.
You've only had them for about month. Most rats take about a month or two(sometimes longer) to settle in a new environment before they start to breed. I imagine, they'll be breeding very soon if they aren't now.
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