Gotta - or someone - correct me if I'm wrong... The way I understand it is a horse's stomach can hold roughly 10L of feed, and if any more is fed it will empty quicker. So, on that note, we in the realm of 'not too much' for OP's horse. BUT The stomach is the primary place of starch digestion, through enzymes. It empties quickly anyway - roughly 12 minutes - and hard to digest or too rich feeds do not have time to be properly digested. It is the undigested starch that causes most probs in the hind gut. The more feed quantity, regardless of type, added to this only means it's less likely digested before it leaves the stomach. It's not the overall amount, or the 'roughage' ingredients of the feed as such, but combined with high sugar, any big meal is more problematic.