I think that's because two of the most important problems with food are quality and quantity that people eat. I'm general terms we eata lot of bad food, so if you have the same meal (so same quality) eating less (or only what your body needs and no more) that should be healthier.

I think I get your point though and eating well is a lot more that eating good food in adequate portions, and I agree, but for the general population over eating and junk food are the worst problems right now imho.