Asklemmy

I don't think there's a single objective answer.

The gist would be that it's habitual due to long term cultural patterns, the same as what any given culture has in regards to food.

At some point, the concept of big piles of food being the default crept in. I suspect that it originated between feast/celebration foods and the "working man's meal" where early workers in agriculture and industry needed a shit ton of calories to keep doing their work. Once enough people see that kind of portions often enough, the mind decides that must be what everyone is supposed to get on their plate.

Then, as things like machinery and eventually robots removed more and more of the physical labor from jobs, sizes never went back down because the outcome of eating beyond what you need isn't immediate and obvious. So you follow the defaults, do what you have seen and internalized as the norm.

But there are still plenty of jobs where loads of calories are necessary to get through a shift. So people still see that, and thus expect it on their plate even if it isn't a healthy amount for the job they have.

TV just follows society most of the time, so a show will most often mirror a norm without any effort to correct for what's best for individuals.

When it comes to restaurants, there's an extra later though. Even if people know they don't need that much, there's an expectation that if you pay for a given order, you'll get the same amount as anyone else that orders it. So restaurants have to scale to what at least a decent sized segment of the population expects to see. They won't be happy if they get less return on their plate compared to another diner, despite that other diner being on a road crew busting their ass laying pavement in the sun all day and needing more.

If a restaurant either changed sizes per customer needs, or charged an extra amount for people with higher needs, they'd go out of business fast