You really need to WEIGH the feed. A scale is your best friend. A "Scoop and a half" could be anything. I know you board.. but the hay being fed should be tested and I know if the BO won't then you are outta luck there.
If you test the hay you will know what that value is and how much you need of 'other stuff' to balance the ration and how many calories you are feeding.. and how many of those calories are digestible. If nothing else you need to weigh the hay and try to figure out how much your horse consumes.
As I said in another post.. the size of "flakes" of hay are determined by the feed in the windrow (amount going into the baler, stem length and type of grass, legume or mix), the ground speed of the baler and the RPM's the baler is being run at. I baled a LOT of hay.. and I know it varies a LOT because I baled hay to optimize the baler efficiency, not the consistency of "flakes of hay." We both tested and weighed the hay so we knew how much was being fed and how much was needed.
It all boils down to calories and metabolic output. Just like with humans. If you consume more calories than you burn, you gain weight. If you burn more calories than you consume, you lose weight. You need to calculate (as best you can) both halves of that formula.