I always forget how to do this and always end up battling Google trying to find the reference for it. This page is a really good reference of all the command line options for 7-Zip, but just for my purposes, here’s how to set the compression method:
-mx0 : No compression (copy mode)
-mx1 : Very low (fastest mode)
-mx3 : Fast compression mode
-mx5 : normal compression
-mx7 : maximum compression.
-mx9 : ultra compression.