The marginal costs to a restaurant of a small amount of additional food on your plate is minimal. The downsides aren't that great either...assuming the price to the customer of a bit of extra food is the same, the people who want extra food will be happy, and those that don't just don't have to eat it.
Now ask that same restaurant to start serving smaller portions, in comparison with their competition. They haven't really cut too much in costs, but suddenly they're the guys with wimpy portion sizes, while the restaurant next door serves up a full, satisfying meal.
I think we're just seeing the result of a market that's catering to the unfortunate preferences of us, the consumers.
This is only true at cheap restaurants. High quality food does not come in large portions. Stop going to garbage restaurants.