Compress how?
Toast is like Nero -- pretty lousy at anything but pure burning. Sure, it offers to do other tasks, and claims to do them well. But it does not.
Do you have a DVD-Video, and need to shrink/compress it to fit a 4.48gb single-layer DVD-R/+R?
If DVD-Video, there are some Mac-native options for that process, better than Toast, but I actually prefer to use WineBottler and install DVD Shrink. MacTheRipper to extract the source ISO, open ISO in Shrink. That is honestly the best method.
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http://winebottler.kronenberg.org/
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http://www.mac-the-ripper.com/
- and shrink is attacked at:
http://www.digitalfaq.com/forum/dvd-...dvdshrink.html