These ISOs are easily tampered with. Malicious actors may inject rootkits, keyloggers, or backdoors into the system image, giving them unauthorized access to your PC from the moment of installation.
The idea is compelling, but the numbers simply don't add up. A fully functional, 64-bit version of the Windows 7 Ultimate operating system requires a significant amount of space: Windows 7 Ultimate 64 Bit Highly Compressed 10mb