The "STEAM" and "SAGE" folders contain the libraries for Spectrasonics instruments. STEAM is the library folder for Omnisphere, Keyscape, Trilian, and all Sonic Extensions. SAGE is the library folder for Stylus RMX.

The size of the STEAM and SAGE folders will vary based upon: (a.) which instruments you own, (b.) if any Sonic Extensions or third-party sounds are installed, (c.) if your instruments are up to date, and (d.) the computer platform you are on (Mac or Windows). 

The primary reason Mac and Windows have different folder sizes is that on Mac, 1 GB equals 1000MB, whereas on Windows, 1 GB equals 1024MB. The factory sizes for each library subfolder within the STEAM and SAGE folder are listed below for both Mac and Windows (rounded down to the nearest GB).