Space on CD-R

Whenever I burn a CD-R to take backup of my data the entire space on the CD-R is never used. When I burn around 250-300 MB of data after checking the property it gives the message that there is no empty space. But when i check it with Nero CD burner it shows that adequate space is still there. So where this remaining space is used. I am using the default CD-burner that comes with Wndows XP. I have also tired Nero CD burner. I burn the CD in multiple sessions. Is it because:
1. I am doing it in multiple sessions, or
2. The default CD burner of Wndows XP is not enough.
Can anyone explain me where I am going wrong, so that I will correct myself.
Thanks.
Rajan