This may be just because they needed to shave the cost not just becuase of the plan.
I think it is more complicated than that. I think the plan is just one of many indications that guests desire more affordable sit-down dining choices. People simply want to eat-out more these days than they did many years ago. The Dining Plan helps address that a little, but I think generic changes to the restaurants have also addressed the desire for a less costly dining experience. Ten years ago, the Epcot restaurants were mind-blowingly excellent, and rather empty at times, but also
very expensive. Now, they're almost all busy all the time. Some are still great, but many of them are much more affordable than they were before, and as a result costs were cut to reflect the lower price. With most things, price is based on value, and that works both ways: If you raise the value, you should raise the price; but also if you lower the price, you should lower the value (if it reduces your costs).
Another thing that is going on, which is perhaps a secondary effect, is that the clientèle has changed a bit. Previously, guests would see a big portion and perceive that as "overwhelming value". Now, more and more, guests see a big portion and perceive it as "let's share and cut our costs in half". Big portions in restaurants were never ever ever intended to be an invitation to share. They were considered a way to trick the customer into thinking that the meal was more worth-it than it really was.
Doubling the size of a portion only increases the cost to provide it by 18%. With the change in the way guests view large portions, it is essential that restaurant reduce portion sizes back to a reasonable size, so they can safeguard their pricing model, which was always based on the expectation that most every guest (over the age of 9, these days) would order a full-priced entree.