Medium rat too big?

[MisterKyte]

So since hes been on sm rats could that be why hes not really gaining weight?

Ove only had him 3 weeks.and ive just been feedong what his previous owners were.
But i think hes big enough to go to mediums.

Three weeks seems a tad impatient if you ask me (give him a couple of months, that's when you start to really notice the weight gain) but yes, feeding a snake inadequately sized prey will hinder its growth.