For 7zip
1)Add Mod folder(best) or contents to archive from context menu or manager
2)Select "Ultra" or "Maximum" in Compression level
2)Select "LZMA" or "LZMA2" in Compression method Spoiler   
3)Check the difference in "Compression Ratio". Less means better.
   


For Winrar
1)Add Mod folder(best) or contents to archive from context menu or manager
2)Select "Best" in Compression method and check the "Create Solid Archive"
(you cannot update your archive but it gives better compression)
   


3)Goto "Advance" tab->"Compression"->Select "Text Compression" to Auto Spoiler   
   


NOTE: Compression ratio depends on CPU, above file size may vary

WinRAR *one* of the best for compression. I use it to store large libraries between my office PC and dedicated server.

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