Horses are supposed to be eating constantly- I think nutritionists say they need to eat for like 18-20 hrs a day O.O They should only be eating roughage constantly though, not hard feed. If you are worried your horse is eating too much and may eventually become overweight, work out how much food she should be eating and weigh it out in haynets. If she gets through it too quickly, put hay in double nets or the super small hole ones.
Horses need, on average, 2% of their body weight in food a day to maintain their body weight. Some need more, some less, and most will need more come winter. So a 700kg horse needs 14kg of total food (mainly roughage), 800kg horse needs 16kg roughage etc. daily. The weight 'calculators' on the internet aren't really very accuarate but they'l give you a general idea of how much your horse should weigh with regards to breed, height and condition. A weight tape is a good (cheep) investment though for if you want to keep an eye on weight fluxuations.
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz