Last I checked, Expansion Packs are downloadable content. Thus Expansion Packs are by definition DLC.
Also size isn't a defining factor. Dragon Age: Awakening was called an expansion pack and was 1.9 gigabytes. Mass Effect 3: Citadel was called a DLC and was 4 gigabytes.
The terms are interchangeable.
I worded my original post a bit better. Check it out
Also, 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 call everything 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 however are DLC. Those just add new weapons and items to the game. Just like the TW3 DLCs like the new crossbow, armor set etc.
I guess it doesn't really matter anymore. Unfortunately. Because, the term "DLC" has been established as the marketable word for anything that is added to a game (and thus plastered over any kind of addition), whether it's an expansion or just skins.