Download and install 7zip if your on a windows server and then find the file and right click > Add to archive. Select archive type of 'Bzip2' on the 'Ultra' setting and then save the file to your 'maps' folder. The server will then push down the compressed version and play with the decompressed versions. If you happen to have a 'sv_fastdownload' host then place the zipped files on that host in the maps folder and your done.
If your on a linux host you can use the 'bzip2' command line to zip the files from the command line via SSH.
bzip2 -z -9 filename.bsp
Will pack the file as filename.bsp.bzip2 in the dir you run from then just copy that file into your maps folder on the game host or the sv_fastdownload and your set.