Before internet even existed, game expansions were always addition to an existing main game. Some required the main game, some were stand-alone. They added more story/missions/maps/chapters/etc to make the main game longer by means of a continuation/intermission/side story. These expansions were always big and added hours of extra NEW gameplay.
When the term DLC appeared, originally these were tiny additions to a game. More weapons/vehicles/skins. The main game didn't get longer or bigger with these DLC, it just made them look different or given new items to play with.
But now that publishers are using the term DLC to promote expansions, the line has become too blurry and people calleverything DLC.
For example, take Deus Ex Human Revolution: The Missing Link. This is an expansion, not a DLC. It adds more story and gameplay, making the main game longer (even though it is separately played from it).
The tactical and explosive packs are DLC. Those add new weapons and items to the game. (The Tracer Tong mission is so short and small it can not be considered an expansion IMHO. It's like only 3 medium areas with minimum enemies. You can finish it in 5-10 minutes)