Expensive meals tend to be higher quality and less quantity. Therefor, most people will complain about 'not being full' when leaving. That's not the restaurants fault, they served you a proper helping most likely. Its the fact most people eat far more than a recommended helping for *WHATEVER* reason. Also I've found people who frequent fast food places, and generally eat allot of junk, don't 'taste' food like people who control their diets.
My suggestion is always; if you want to be full, go to a buffet. If you want a real meal, go to a high(er) end restaurant. Keep in mind all those salty cheese burgers you've been pounding down, won't do your taste buds much good when you go to a nice place, and its not the fault of the establishment.
I think the preconceived notion is; go to a fancy place, have a ton of great food. Well yes, its great food, but the portions are well correct, and that is off putting to allot of folks who tend to eat more than what they need or is recommended.
I will concede that some of the nicer places at WDW have a bit to be desired lately. To the point i look at the menus before I go somewhere, to make sure there is something my taste buds will go crazy over. Unfortunately, with the decline in plates, the costs have not sunk with the quality, and I can see where this would a few people off.
I like to think of Disney dining as, you pay for the next tier, but get the tier below (compared to the real world). So .. a spendy place at disney, probably isn't as good as a spendy place outside of disney. Its pretty simple.