Hi Hamit.  Your disk space isn't as acurate as you would expect because as your disk gets formatted for installing windows, data such cache and other formatting gets written to your disk in addition to your windows files.  These take up space, so your disk of say 1000GB has already got a few gb of it's space occupied.  Also you must take into account that if 1MB = 1024KBS and not 100KBS as is usually expected, then space taken up by formatting and windows is far more than expected.  So if your installation occupies say 2GB of space, then 2048MB of your disk space has already been taken, hence your perceived discrepancy.  And last but not least, different Operating Systems, for some reason have slight differentials to their way of calculating disk space and applications sizes.  Mac and Windows are just two such examples.  Hope that helps.  Cheers!