Limiting the hay a bit more is also a good point. 1.5-2% of the horse's ideal bodyweight per day in forage is the recommendation.
My 15hh, 1000 lb horse gets 4 flakes from a small square bale daily- the flake size can vary a little bit, but it usually averages out to ~20 lbs total.
A slow feeder net is a great idea when limiting hay, as it keeps them eating longer which is better for them mentally and physically. I fill one up for my horse whenever I see him, but unfortunately the feeders where I board won't fill it up for me