The short answer:
If you’ve come across search results or forum links promising a “Windows 7 500MB ISO,” you might be intrigued. After all, a full official Windows 7 installation DVD is around 2.5–4 GB. How could anyone shrink it to just 500 MB?
If you need Windows 7 for legacy hardware, use the official ISO and apply the and SHA-2 support updates manually after installation.
A 500MB Windows 7 ISO is almost certainly a stripped mod or a trap. For real-world use, stick to official ISOs or lightweight Linux distributions (like Puppy Linux or antiX) that are designed to be small, secure, and legal.