You want to know what is amazing, that Far Cry 2 is about 2 Gigbytes while Far Cry 3 is 15Gigs?
How the heck Far Cry 2 is only 2 Gigs?
Actually the installation size of FC2 (Fortune's Edition, GOG) is 3.29 GB, uncompressed. But yeah...
Also where did you check for the FC3 being 15 gigabytes in size? The HD requirement specs in e.g. Steam pages usually seem to put little extra to the actual installation size, but I guess someone who has FC3 installed now can easily check the actual FC3 directory size that Windows reports.