Does the hay not get dirty? In the winter, half the piles would get snowed on and disappear under the snow before the horses got to them. And some of it would get blown away (we are on a hill).I really feel, and see with my own horses, that small, small piles, but lots of them, makes the least mess/waste. And mine are not on grass at the moment, nor do they get grain. Like you I give grass pellets and the supplements twice a day.
Right now, they are happily dozing under the trees. They will finish the little bit of hay left after their afternoon beauty sleep.....lolol.....then I throw more tonight.
Like most thing with horses, it is trail and error when it comes to laying out hay. But between the boarding barns I have used, and having horses on my own property, currently and fought 25 years ago, I really find the many small piles versus four big piles works wonders and keeping an area clean (mine have their two poop spots away from the piles of hay) and horses not pigging out.
I've also NEVER seen my horses sleep when there's hay available! LOL I'm telling you, mine are pigs and never seem to think they've had enough. I intentionally don't give them any hay for a few hours in the afternoon so they'll sleep.