Why is file too big for disc??

I am trying to put scans onto a CD. The file says that it is 703 MB. Why is the Wizard telling me that the file is too big to put on a 800 MB CD?
Windows XP.
It told me I had to get rid of a small (under 100 ) amt of KBs, so I deleted one scan. Then it acted like everything was fine so i left, & then it says again that it cant burn the file because it's too big. What I deleted was 735 KB, so it should have worked.
Thanks.

 

I thought is was because some space on the disk is reserved for the table of contents, it might be like a hard drive, the actual capacity of the unformatted disk.. read up here ..http://www.osta.org/technology/cdqa7.htm
It changes depending on the format your using too.