Warning: Some highly compressed versions cut the Titans expansion to save 100MB. Always read the file description or NFO file before downloading.
Highly compressed (or "repack") games are standard game files that have been run through advanced compression algorithms (like FreeArc, LZMA, or KRK). While a normal installer just copies files, a repacker uses tools to shrink audio, textures, and animations down to the smallest possible size.