There are many reasons why your cockatiel might be eating more than usual, but most of the reasons are nothing really to worry about it.
If your cockatiel is very active, is laying eggs or is feeding young chicks, your tiel of course will eat more. Also, a low temperature in the environment will make a cockatiel eat more.
"If your cockatiel is eating a lot but is lethargic or you don't see a visual gain weight it could be a sign of illness. Check your cockatiel droppings to see if they are watery. Parasites can also make your tiel eat more than usual."
Did you know that a cockatiel can also suffer from diabetes?
This could also explain why your bird might eat a lot but still acts lethargic. In cockatiels with diabetes, the problem evolves from the pancreas being damaged by infection or other secondary issues.
If you suspect that your cockatiel might have a serious illness associated with eating please do see an avian vet.
As always the information offered here is to provide guidance and is not intended to be a substitute for the good advice provided by your own avian vet. When in doubt always consult your own veterinarian.