Video game files (textures, audio, cutscenes) are often already compressed using algorithms like LZMA or stored in proprietary .iwd files. Re-compressing a file that is already compressed offers very little reduction (usually 1–2% max).
The storage requirements for MW3 vastly exceed 100MB, making such a download impossible for the actual game:
If you decide to ignore the math and click on the "Download Now" buttons found on YouTube descriptions or sketchy warez forums, here is what statistically happens next:
: These downloads frequently contain trojans, spyware, or "rootkit destroyers" that can cause complete system failure.
Instead of a virus, the file might just be a "downloader" that fills your screen with endless pop-up ads. It changes your browser homepage to a fake search engine. Your PC slows to a crawl.