Wrong used space on cd

Archived from groups: microsoft.public.windowsxp.help_and_support (More info?)

I am copying files to a cd-rw and when I look at how much space has been
used, the computer only shows me the size of the last copies I have made,
even thought all of the files I have copied show. For example, I have used
about 300 mb of space on the cd. The last thing I copied was 78.9 mb in
size. When I look at the cd, it says I have only used 78.9 mb of space.
How do I get an accurate reading of how much space I have used on the cd?