The size of Elastic Block Storage (EBS) on Amazon EC2 is charge by the size of the formatted drive, not the size of data on the drive. In the case of the Windows-Server2008-i386-Base-v101 this is a 30 Gigabytes (the primary hard drive). Which you pay for regardless if you are using all of it or not.
This is different the the dynamically expanding .vhds that are used in Windows Virtual Server.
EBS storage is allocated all at once to avoid disk fragmentation and poor storage.