+1 to Alex and Pit.
I don't give our boys anything bigger than a small, and they still put on weight fine. In the case of Mr. Bubbles, a little too fine. He's now pushing 1600 at just under 3. Upside is his breeding drive is strong.
Our girls, I'll offer a medium rat now and then if they seem to be growing a little slow (we aim for 500g a year for our girls for their first 3 years, so if a 2 or 3 year old is lagging behind but eating steady, I'll bump a meal for them once a month or so) or if we're trying to encourage ovulation.