They grow just fine on mice or rats. Sometimes they go through a growth spurt and will stay long and slender for awhile, before filling out. 2 mice a week is just fine, especially if it's a male. All 35+ of mine are on mice(1-4 depending on how big they are and if they are preparing for breeding). They grow and reproduce wonderfully. The adult females have gained all of their weight back plus some, and they laid late clutches this year(May/June).

When they go through those growing periods, they look odd. Just long and gangly, like teenagers They fill out after awhile though.

And no worries, both of my Yellowbellies are spitfires and won't hesitate to strike at the front of the tub at me.