In a standard ZIP, each file is compressed individually. In a "solid" 7z archive, all files are treated as one massive data block.
Unlike some proprietary formats, 7-Zip is open-source and widely considered safe for distributing large software packages.
If you have many similar files (like versions of the same document or many ROMs from the same console), 7-Zip can find patterns across all of them, resulting in a significantly better compression ratio.
The .7z format, created by 7-Zip , uses a method called to achieve much smaller file sizes than traditional formats like ZIP.
Most modern tools, including WinZip and various emulators, can now open or at least read the content of these archives directly.