I believe there are a bunch of game assets pertaining just to people who bought the expansion. From the top of my head, I can think of:

- 3D Maps (& their textures): the new areas, dungeons, cities, doodads, etc (those take a VERY BIG CHUNK of game data, arguably the majority of the expansion's data)
- Icons for the new classes
- NPC data for the new areas
- cutscene data and voice acting audio (on multiple languages, one must remember) for lvl 51+ Main Scenario quests.

I don't believe these are downloaded to non-expansion players. This is game data you're actually licensing to possess and use, and downloading, when you buy the expansion.