Because, PDF is a container, like a fancy zip, the pictures saved in are kept in their original format, (JPEG, PNG/APNG, AVIF, GIF, JP2k, JPXL, BMP…) unless you add a step to re-compress the all document with image.
It’s always slightly bigger than the original file.

1. you need Ventoy to stop formatting you’re USB sticks
2. Keeping lot of ISO is a bit useless just the few that you use daily.
3. If you’re keeping this ISO anyway, get them by torrent and keep sharing for helping the community