I just eat enough for enjoyment and never beyond that point. I have no problem leaving a plate still half loaded.
This, though I accept that many people, particularly of the older generations, had this ability beaten out of them at a formative age.
Ultimately, the food is a marginal cost for a restaurant, and you rarely get bad reviews for severing too much of it. Stands to reason that portions will tend to inflate to USAnian proportions.
The US attitude to taking home what you don't eat is theoretically a good idea though the expectation that you eat till you feel like a blimp and still have enough food to take home a meal has fuelled portion size inflation even further (because people would genuinely complain if they arrived at the end of the meal to an empty plate). Generally, if you eat out with any frequency, you end up a fridge full of boxes and an increasingly parlous game of food roulette.
I'm from the generation that had to clean it's plate, my mother was never joking when she said
if you don't finish that you're getting that tomorrow. Given her cooking skills that was a grim fate. Mix that food parsimony with US portion sizes and you're soon going to need a pick-up truck to get around.
Any decent restaurant where they cook from scratch, you can, of course, ask for less food and they'll have no problems with that – but I imagine any chain place is mostly just reheating or frying pre-portioned food, so they may not accede. Personally, I'd rather go to the former where there's someone in the kitchen who can and actually is cooking ingredients rather than reheat catering supplies.
