That includes disk space for several third party applications, swap files, cache files, etc. As far as how much space my Yosemite installation occupies the breakdown is…
  • Apps 186.05 GB
  • Movies 154.32 GB
  • Audio 129.3 GB
  • Photos 36.6 GB
  • Other 71.7 GB

A standard TechTool Pro 8 eDrive is 15 GB of which 7.14GB are used by the minimal OS X and TTP 8.

ProToGo configurations including the total space allocations are all user configurable. Of course you would not be permitted to create a ProToGo volume that is smaller than the selected OS and applications require and you would also want to leave room for swap files, cache files, etc. if you want the system to be stable. I have not attempted to build every ProToGo installation but it appears the default space allocations are on the order of 50% for the system+TTP8+selected apps and 50% free space.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein