DVD Drive

Hey Geeks, This one really has me stumped. I am using a Toshiba DVD-ROM
SD-R1312. I am using Memorex CD-RW disks that state they have a capacity of 700mb at 4Xspeed and 80 min. My OS is Windows XP Pro. I only have a 30 gig hard drive and lately I have been trying to increase the space on this Cdrive by copying files and programs over onto these CD's. When I go into "MY COMPUTER" my "F" drive-the above mentioned DVD drive usually starts out reporting a capacity of -for example- 638mb. As I use these disks and start to transfer the files over from my "C" drive the "F" drive keeps reporting less and less capacity such as 600mb, 540mb, 388mb etc.I am not refering to the 'free space' that is available but the total capacity.The current disk that I have in the drive at the present time is reporting a capacity of 98mb.(that is from the original 700.}and therefore when I continure to try and transfer more files the recorder reports that the transfer is not possible since there is not enough available space when in actuality there certainly must be lots of space on the disk.
All help appreciated. Please be specific. Thanks,JamesP4056.