One time, using the Linux tools and both Rockridge and Joliet extensions for the filenames (All horrible long filenames with spaces and crap...every file) I burned some 690 + Mb of files to a Verbatim 700 Mb DatLifePlus CDR disk.
bash-2.05$ du -s /mnt/cdrom 711431 /mnt/cdrom
Value is in kilobytes. I was kicking myself for not doing a test burn (-dummy mode, it's called in my software), thinking for sure I was going to make a coaster as it was displaying progress, but nope! There must have been extra space on that disk. I can't believe how much data I crammed on it.
I don't think I'll try my luck again like that, without testing first though.