That said, people in general do tend to eat too much. I'm not even just talking about processed and "junk" foods. Throughout most of human history food was very scarce and we can go a shockingly long time, by modern standards, without eating.
Speaking from personal experience, the less I eat the less hungry I feel and therefore the less I want to eat. From my limited understanding, this is hormone related. Insulin is the hormone that our pancreas releases when our blood sugar spikes and it's our body's way of saying "there is energy here to store." Once the insulin spike drops you start burning stored fat and your fat cells release another hormone called ghrelin which triggers the hunger response. So as unintuitive as it is, eating less is actually a good way of feeling less hungry in general (after the initial hormone spikes have dropped). I've also heard that physical exercise can act as an appetite suppressant, though I don't know why.
So you're absolutely right, if people made healthier lifestyle choices in general (and knew what those choices were since there is so much contradictory information out there) then that would reduce demand on the food supply chain.