Sometimes feeding them prey that is too large will cause them to vomit up the meal a couple of days later. You will have to use your judgment to determine how much to feed them. They tend to eat more than is good for them if given the chance. We feed ours small to medium-sized rats, one or two per feeding depending on the size of the rodent. We use canned cat food occasionally, but we don't offer it as a staple diet.
Cat food is very rich and fattening and can cause diarrhea.
Our monitors seem to also like ZuPreem Tegu and Monitor food. We feed this to them only on occasion, and it doesn't seem to cause the runny stools like cat food. Our monitors diet is also supplemented with crickets. They seem to enjoy them, and chasing the crickets around also encourages them to exercise. If Savannah monitors are not allowed to exercise enough and fed too much, they will get VERY fat.