Nah...I ate more (quantity wise) and more regularly the last time I was at WDW, than I normally do, and I lost weight. It's the stupendous amount of walking and the constant hydration.
I'm not sure of your background so I'll explain roughly. Calories are a measurement derived from how much energy it takes to heat up a specific volume of water 1 degree. So as you're walking all over WDW (effectively exercising, but I don't think anyone really has as much fun at a gym as in WDW) and drinking water and juice, and ice cold sodas, well all those cold drinks have to be heated up to 98.6...and although the external temp helps some, that's still more work your body has to do. So if you stick to water...which is has no added calories...you're really burning the calories over the length of a day.
And for pigging out in the Magic Kingdom or at the Poly...Dole Whip. Not only are they delicious as all get out, pineapple is one of several fruits which contain an enzyme that helps break down proteins...so you won't feel stuffed for so long.