How can you tell just by size? Both of my monitors were rescues and were starved when I got them. I am certain they would have been bigger if they were properly cared for in the beginning, especially my Nile, he's a tiny guy at two years old...
About two months ago Chris (Mumps) told me to start feeding them every day, and I did right after he said so. They won't always eat every day, but usually they will take it. I have also since changed their diet a lot, I no longer give hard boiled eggs or beef liver to them at all. I actually stocked up on a lot of canned foods that are meant for monitors, I got ZooMed canned monitor/ tegu food, canned grasshoppers, canned snails, and canned shrimp. I also feed live dubia roaches, rat pups, and super worms, so they are not just eating canned stuff. I have to admit, they are a bit more active now, so a special thanks to Chris (Mumps).