http://gettyequinenutrition.biz/Library/Theoverweighthorsewhowontstopeating.htm
You haven't said if yours is a mare or gelding. My mare will be more hungry when in season and a little rude snacking on hacks as well.
I have only heard of what the above article mentions, that restricting food causes the metabolism to slow and go into survival mode. Also reducing food causes a stress hormone to be released which causes weight gain.
Have you ticked the boxes for any metabolic problems? We ticked four out of five and she went on to lose the weight. I did it whilst on box rest I admit but the vet needed to see she could actually lose before we went down the testing route. She was also fed adlib to begin with and gradually reduced.
There was a documentary on in regards to sugar. I am shocked by how much sugar goes into horse feeds let alone ours. They couldn't decide if it was addictive. But as I can polish off a large bar if chocolate long after my dopamine is satisfied I would say it is!
Your horse might be greedy because they are addicted to sugar. Cut out