The executable setup files inside these compressed archives often contain hidden malware designed to steal personal data or enlist your PC into a botnet.
Many sites offering these files distribute malware. Security scans of "highly compressed" AC3 executables often flag them as malicious (e.g., Trojan.Genome). Assassins Creed 3 Highly Compressed 500mb For Pc
To understand why a 500MB version of Assassin's Creed 3 is problematic, you have to look at how modern video games are built. The executable setup files inside these compressed archives