Steam only has had the 5 GB download. The 1,5 GB was what people got from GamersGate and Impulse. I wonder if the actual game download only is 1.5 GB, and the 5 GB is the incorrect Steam version that then replaced the files from the 1.5 GB download (as the files are different). From what I saw at the Kerberos forums (when they briefly worked) was that the release version is supposed to be around 2 GB because it is properly compressed (the audio and graphics), which wasn't the case for the beta build. I am not 100% sure if this correct since I wasn't in the beta and I don't have access to the proper release version, either.