Why the hell is it so huge when it's mainly bug fixes?

I assume it contains all the assets for the new content, so people who didn't buy the DLC can still play with the people who did.

It is because it contains the DLC bits from the expansion, and because of the controller support.
What the update has brought are only troubles. TQ Defiler can't read characters overrtten in the latest version, some players report malfunctions in the TQ Vault, and regarding patches, update didn't address the bugs that are present in the game for years now.