The study this article is about confirms what I had already thought, although plus a bit of more sciency detail! It is obvious that if they get hungry they will then eat a lot to compensate, although there may be ways around this such as using a paddock with poor quality grass, or stabling and letting them eat straw if they will.
I also suspect that if they are kept off the grass for periods of time with nothing else to eat then it also messes about with their metabolism, and when they do have their grass binge, they store the weight.
Interesting point in the article about the ponies learning to moderate their eating habits over time, this is also something that I had thought was happening at our yard when we only had every other day turnout for a while.