Short answer: the amalgamation of “modern” French and English as well as a shift in eating habits, [particularly in North America](https://en.m.wikipedia.org/wiki/Supper) led to the change in phraseology around the start of the 20th century.
Longer answer: “Le diner” means “to eat” in French. Supper comes from “souper” meaning “to eat” various forms of French outside France, particularly in places with English presence (like Canada or Belgium).
Dinner *used to* refer to the main meal of the day, lunch. This is because dinner/lunch fueled the grueling labor of fieldhands and industrial production. Your later meal was lighter, usually just to hold you over till breakfast.
Different parts of the world refer to lunch/dinner and dinner/supper differently, but this can generally be broken down into Anglo versus French backgrounds. I grew up in the Midwest near French colonial towns — lunch was lunch, dinner was dinner.
In the south (where I now live), dinner and supper are used interchangeably. When I did my study abroad, the north of England called your later meal “tea” but understood when I, a Yankee, asked people to go to dinner.
This, of course, has nothing to do with actual *amount* of food consumed at all these various meals. Tea was usually a large affair for me and my Northern English chums (to say nothing of that sweet, sweet Full English brekkist), but my college friends and I generally ate more at lunchtime because we were studying late into the night.
In any case etymology suggests the shift happened in the 20th century as mass communication began, just before WWII.
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