Before diving in, it is critical to understand that "highly compressed" does not mean "game breaking." There are two primary methods of compression used in the emulation community.
Downloading and using ROMs for the original Xbox can be a great way to preserve your physical game library and enjoy classics on modern hardware through emulation. Because Xbox games were originally stored on dual-layer DVDs, files can be quite large (up to 8GB), making "highly compressed" versions very popular. Understanding Xbox ROM Compression xbox roms highly compressed