I don't know what kind of disc you're trying to burn here, but if it is a data disc then you can just mount it, and burn a Data disc using the files from the mounted image file. Fill the disc with as many of the fiels as you can fit on there and then burn a second disc with the remaining files on it.
If it is an audio disc then you can rip the songs to your hard drive and then burn all but one of them on one CD and last one on another CD.
Or if it is Video then you could mount it, open Toast, and burn the disc from the Video_TS folders that are in the mounted image file.
In other words, you can get around the problem using other methods. Toast won't compress an image file to fit, but (for video) it will compress the contents of the image file.