In my experience, the standard serving size at Thanksgiving is "cook for an army", and that's if nobody brings anything.

People fall all over themselves to make you a plate, make your bedridden neighbor a plate, make your neighbor's cousin's doctor a plate - just to get rid of all the food! Plate out a meal, cover with foil, repeat until all food has been divvied up to be taken home and stored in somebody else's fridge, and, ultimately, belly.