At least, that's what it says when I go to my library and try to download its installer. That can't be right, of course. The DLC's Steam page says it needs 32GB, which makes much more sense, but it doesn't help me here. What's going on here? What can I do?

I think the files are already included when installing the game and this DLC installer just unlocks it so it can be played.

What if I installed the game long before the DLC came out?

And in any case, how exactly do I start the DLC? If I know how to start it, I guess I can test that out to see if I have it. I did run the installer for it, so if you're right, I should have it, but I don't know how to start it. Do I need to start the whole campaign all over again?

Originally Annihilation Line DLC was the size of what you would expect. Most likely when the DLC was released that the published decided to streamline the main game and the DLC to make it easier for people who purchase the DLC to unlock it. There is something good to say about this but also something bad (way bigger file map) but you are not being ripped off. Annihilation Line was not finished when Terminator Resistance came out.

As for how to play the DLC, you probably have found it by now but just in case you can find it under "New Game" as it is a separate campaign from the main game even if it starts halfway during the main game.