Less than 10-15% for a ball python (a generally inactive snake) can cause weight loss. To maintain body weight, a 10-15% intake is required. To add weight is 20% or more (for malnourished or baby snakes). The snake knows that if it doesn't eat at all, it will starve. If it eats less than what it should it will stay in hunting mode longer. So you're telling me that you have a ball python, and because it is inactive, you don't have to feed it? How do you calculate how much your snakes eat? What you believe is just enough to keep them going? Just like the AVERAGE caloric intake for a human is 2000 calories, some need more, some need less to survive. The difference between me and Monty is that when I feel hungry I can walk into the kitchen and open the fridge. He has no way to tell me he is hungry other than to strike.
I'm not sure snakes do know that actually...
I was told that if they can't easily curl up in a tight ball, they are overweight.