I know how you feel. I was feeding my girl small hoppers when I first got her at around 2 months old thinking she wouldn't be able to take larger prey. Then I watched one at the store only 2 months older nail and take down a medium adult mouse with no problem. Needless to say I started the switch to larger mice.

She's 6 months old now and eating large adult/small jumbo mice one a week and slowly catching up to where she should be weight wise. The key is to judge by the widest part of the snake, not the head or neck.

Good luck getting yours up to speed.