Here's how I see it. Some decades ago, fast food restaurants are looking for ways to compete. "I know," they think, "consumers love getting more! Let's up-sell them on items that give twice as much food for a 15% price increase. It's a good deal for them and it's good for us because our main costs are fixed (ie, labor, building, equipment, and other things that don't change in price much by selling more food in a single transaction.)"
Or you get trends like candy bars, which back in the 80s started getting bigger without rising in price -- more is better! And once one company starts doing it, all the other companies have to do it to stay competitive. You don't want to be the stingy restaurant who serves tiny portions.
Except that many consumers don't actually want MOAR. And eventually some of them point out to the restaurants that their portions have grown to three times the size that an person of average proportions needs to eat in one sitting, and maybe a smaller portion would be nice?
But a restaurant can't shrink their portion sizes! That's crazy talk. How are you going to advertise "Come to McD's and get LESS for your money?" Yeah, there's a winning campaign. But some consumers really do want less, and you don't want to ignore them ... aha! Let's add a smaller version of our existing menu items, and call it a 'snack'! Okay, so it's the same size and about the same nutritional content as our 'meal' items 30 years ago, but we can't call it a meal because it's not what people are used to seeing called a meal. It looks all scrawny and small on a plate.
It is, however, the right size for a meal and many people realize this. So they start eating the 'snack' sizes instead of the gigantic 'meal' sizes. And they call it 'snacking' because that's what the menu says it is. And thus the trend comes full circle. And only the language suffers. >:)
Yes, I'm sure that's not all of it. But I do think it's a significant contributor.