Flame Tree BBQ in AK has huge portions, especially the 1/2 rack of ribs plate and the 1/2 smoked chicken. They are both served with a couple of sides.
Tangierine Cafe in Epcot's Morocco has an absolutely enormous shawarma chicken and lamb platter. It is served with hummus, tabouleh, couscous, and bread.
Pecos Bill's in MK has a fixins bar (lettuce, tomato, grilled onions, grilled mushrooms, sliced cheese, melted cheese, pickles, etc) for your burger or salad, and you can add lots of those yummy toppings to make a very substantial meal. I believe that Cosmic Ray's has a similar bar, too.
The meatball pizza I had at WPE in DD was scary big. When they brought it out, I said out loud "Really? This is a meal for 1 person?" The waiter said that he heard that a lot.
If you want to splurge on a meal, here might be some good options:
Although it is pricey ($33), I am always 'Thanksgiving full' after a meal at 'Ohanas. All you care to eat of everything. Whew. The meats are good, but my favorites are the dumplings and the bread pudding. Yum.
The best bang for your buck at a buffet would be Trails End breakfast or dinner. It is considerably less than other buffets on property. I just looked at the menus, and Trails End dinner is about $14 less than Chef Mickey's.