A true 500MB final install folder is impossible for AC3. What repackers offer is a 500MB setup file that decompresses to 5-8GB on your hard drive. If you see a claim of "500MB after installation," it is almost certainly a fake or a stripped-down tech demo, not the full game.
Compressing a 17GB game down to (a 97% reduction) is generally impossible without removing core game files, textures, and assets, which would make the game unplayable. Why You Should Be Cautious Assassins Creed 3 Highly Compressed 500mb For Pc
If you are looking to play Assassin's Creed 3 on a budget or a low-end PC: A true 500MB final install folder is impossible for AC3