I could be wrong but it seems that you are closing the CD when you burn the first file, If you want to add more files later, you need to look for a "keep this project open" button/checkbox in your burning CD software.
The idea is that you first put files in the CD and then you need to 'close it', create an index or put some marks so the disk reader can find all files and the CD is ready to be used . A closed disk is finished, will not accept more files (would broken the, lets call it, 'file index'), so there is not 700 megas available anymore, only the size of your first files.
Other possibility could be that your rewritable CD are damaged, or that you are using the wrong type to save your work, maybe creating an iso image, when you do not want this really. Top quality "Raw" image formats are much heavier than jpg or png. Sometimes you need to save as raw, other is OK to lose some quality. Keep trying, is confusing the first times
What do you want to do?
