We hacker/engineering types seem to have an almost visceral rejection of the idea that we are anything less than in total, conscious control of our actions. You see this mostly in reference to advertising ("Oh, I don't see ads, and even when I do, it's not like they affect me").

But the thing is, that's not entirely true. People are not hyper-rational processors of data. We're startlingly susceptible to all sort of subtle manipulation, even when we're fully aware we're being manipulated (See the recent front page link about a guy who was sold $100 dollars in facial cosmetics despite walking in to the situation fully determined to critically analyze the techniques used as a learning experience).

The point I'm trying to say is, yes, you will know you're wearing a headset and yes, you will only have X amount of food in your stomach. But that doesn't mean the subconscious drives that run hunger and satiety (which are far more complex than just "Dry volume of food consumed") can't be tricked by visual stimulus, even visual stimulus the conscious brain knows has been altered.

You'd be surprised. Eating less food when you have smaller plates is a well-established behavior, even though your argument still applies - you can feel exactly how much you ate, and you know that your plates are unusually small.

I can definitely concur the opposite. The more I have in my plate, the more I want to "finish" that meal, and there I end up eating more.

The point is that, despite what you've said, they found that it works.