Can you feed a lizard too much?

Yes, Yes you can! I know for a fact that if I feed my bearded dragon too much he gets a big stomach even when he's not flattening himself out. Just think of it this way:

Whatever lizard you have (gecko, iguana, dragon, etc.) remember how they'd eat in the wild. Some days they'd find a lot of worms, but they'd also go without finding ANY crickets on others. No matter how much they beg or how spoiled they are your lizard has a required amount of food that should not be raised or declined. You also have to balance how much meat they get compared to vegetables but that's another question entirely.