It does vary A LOT from horse to horse. I have some here that get no grain at all, just hay and their mineral blocks. The most grain I feed to anyone is a gallon per day, divided into two feedings--mostly oats, some mare/yearling pellets and some BOSS--I have two yearlings on that ration. They each get that amount. (I would never give anwhere near that much grain to some of the horses, just because grain makes them much too nasty and they'd be out there killing each other!!)
And there are some here that get grain in amounts somewhere been 1 gallon and "none".
I noticed the other day when I was in Tractor Supply they had a bagged feed especially for minis? Anyone know what that ration has different from regular horse feed?
To add my 2 cents worth on the question of amount, when I asked our State Horse Specialist that question while researching for my book, he said that just like any horse to feed the miniature horse according to body weight. Then allow for the individual's body condition, plus or minus.
The mini I had did well on his small pasture except in winter. He got about 1/2 cup 10% sweet feed twice a day and hay in winter or when weather was to bad to be outside. He stayed in nice condition, when he started to gain I cut back. I don't think I ever had to feed him more than that. We added black sunflower seeds, just a handful, in spring to shine his coat. But he didn't really need it, and we quit giving them to him. He stayed nice and shiny without them.