I think a lot depends on what you are planning to do with this horse or your minis....
My breeding minis are run in pastures, guessing each one is about 10-15 acres, and will have broodmares/foals and the stallion, each breeding stallion in a pasture with their herd. Then my yearling and two year old fillies in another, then non-breeding stallions/geldings in another.
They are on free choice pasture (bremuda) 24/7 and free choice minerals.....that's it......all are fat, slick and sassy.
Broodmares are put and grained pre and post foaling, and of course weanlings are grained for a full year......but the rest of my herd live quite naturally out as the grazers they are, since I have pastures.
now onto the show barn....
Show horses are up in the barn, stalled with limited turnout, and are worked several times a week. They each have a specific feed program for them, as far as volume of feed (grain, alfafa, grass hay, beet pulp and supplements) but I have a mare now eating 10 cups of grain (14%) twice a day, pad of 16% protein grass hay in the am, 4 cups of beet pulp mid day and 8 cups of processed alfafa (chopped) at night, and supplements.......and she's just right, she has a high metabolism and is "working" as well.....actually I'd still like to get a touch more condition
So depending on what you are doing with your horse, will depend more on just how much, or at least here it will. A leisure horse out in pasture won't take as much to maintain and a show horse up working of calories.