I think my Savannah Monitor is overweight, what should I do?

I have 2 savannah monitors which I feed like one thawed chicken every other day and a mouse on occaison (chickens are cheaper) I dont put any supplement on the food because I didn't think something that big would need it. One of my monitors is kind of a fatty and moves way less then the other one, but they eat the exact same. What should I do? thanks