Compression levels

What does the compression levels mean?  Seems Standard compression means almost no compression - I have 37gigs used on my hard drive and the image file shows its 32gigs.  About a 14% compression ratio.  Haven't tried the other settings yet.   Any ideas?

 

Also, is there any disadvantages to the various levels -ie. 100% chance of recovery at Standard setting, 75% chance at medium, and 1 in a million at high?  Please advise and thanks for your time.