This makes no sense.
Before you reinstalled, what size did your HD show after you erased? ....and when clicking on the Macintosh HD partition under the HD, how much available space.
ie. On a 320GB HD, it should look something like this:
Capacity : 319.73 GB (319,728,959,488 Bytes)
Available : 319.73 GB (319,728,959,488 Bytes)