In general when referring to a small rat, it is considered to be 5-6 weeks of age, which in my opinion has made your snake fat if this is the size your feeding. The males are pretty big at this age, and can be any where from 100-150 grams.
I feed most of my balls a weanling rat. They are only 3-4 weeks of age, and only weigh 60-80 grams.
Try a smaller prey size or spread out your feedings longer than every 7 days.