Most likely an absurd amount of high resolution audio/video files, 3D models, and textures. Most of the work that occurs in games these days is based on art rather than programming and has evolved to whatever "normal SSD" can be expected to hold
If the game is 55GB, and the update is 55GB, and the game remains 55GB after the update.... ^.^
But if it's a 600GB game? Well then dunno. But I don't think any game has reached there yet... CoD is getting closer though