Do not download any "highly compressed" Windows ISO files, especially those under 1GB. You will not get a working operating system—you will get a headache, a virus, or a bricked PC. Always download Windows directly from Microsoft.
Even the most advanced compression tools available today—like 7-Zip on Ultra settings, WinRAR, or PAQ—can only compress data by a factor of 2:1 or 3:1 for executable code. That would still leave you with a file around .
Have you encountered a suspicious "tiny ISO" online? Share your experience in the comments below to warn others.