To summarize:

WWE '13 features an incredibly deep suite of customization tools, including Create-A-Superstar, Create-An-Arena, and Create-A-Finisher. Saving highly detailed custom wrestlers, custom theme music files, and complex arena logos will write directly to your virtual memory card files, expanding your total storage usage over time. Storage Management Tips for Retro PC Gaming

Once you extract the ISO or install a repack, the game expands.

The small size of WWE 2K13 compared to modern titles is due to lower resolution textures, compressed entrance videos, and the lack of high-fidelity audio files present in current-gen games.