When you use tools like 7-Zip or WinRAR to smash a Windows ISO down to 50% of its size, several things break. A "fixed" ISO includes pre-applied patches.
Windows 7 compressed ISO around 900 MB typically involves using unofficial "Lite" or "Super Lite" editions. Standard official Windows 7 ISOs are much larger—approximately 2.34 GB for 32-bit 3.10 GB for 64-bit Microsoft Community Hub Highly Compressed & "Lite" ISO Options windows 7 compressed iso 900 mb fixed
It sounds like you might be looking for information on a highly compressed version of Windows 7, often found in enthusiast communities. When you use tools like 7-Zip or WinRAR