Before the first DLCs, the expansions were mainly distributed with CDs or DVDs, like the examples given by @bkrueger. And according to this reference, the DLC term was coined in 1997 in the context of Total Annihilation, which had DLC for both new scenarios and items. There might be other claimed origins, though. EDIT: Or maybe it was still only a concept and not the word; the real origin seems to be very hard to pinpoint, and it depends whether it's the term or the concept.I don't think it's hard to understand why so many associate size with it to some degree. When the term was first coined, that's when we started seeing all these tiny DLC (like cosmetics) flood the market. Oblivion's horse armor was the first time I can remember seeing it called "DLC".
Back then, people assumed "DLC" was going to be something smallish. That's obviously no longer the case, but it's stuck with some people.
But either way, things have changed a lot since then. That's what I said: today, there's no reason to separate them any more.
And indeed, Phantom Liberty is called a DLC, but it contains a story line, side quests, a new type of gigs, vehicles, items, new endings, ... The pathfinder expansions are also called DLCs, so are the potential expansion of JA3 and the upcoming one for Elden Ring. Conversely, new tracks or cars for simulators are often called expansions.
Maybe that expansions are (or were) less often used for small add-ons, but I think it's rather pointless to try and argue over words that are used equally for the same things now. It's like the never-ending discussion about open-world, CRPG, or even RPG. We may find a majority here who think a word or acronym has such or such feature - and disagree on other features - only to realize it's used differently elsewhere.
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