[Loren]

Not sure how much you are feeding, but I personally get good slow and steady growth with my garters with 1 or 2 food items per feeding, 1 feeding per week. The really long and slender ones like my ribbon might get 3 rodents per weekly feeding, as its mouth is fairly small for its very long body, as well as any garters that need fattening up. Little babies- maybe every 5 days is better.
But I feed only rodents, which seem to be a little bit more long lasting of a meal. Fish seem to go through them a bit faster in my experience.
Garters that appear nice and thick might get fed less than this, maybe 1 meal a week, or 2 items at once, but every 10 days or so.
But I dont breed either, so my adults dont need as much fattening up.
Garters definitely do seem to have a fast metabolism, but once they get to the point of nice and fat, I think its best to back off a bit, unless breeding. Garters in the wild seldom look quite as fat as many garters I see in captivity.