I've got a couple of DMGs that should fit on a CD (one per CD) but burning softwares (Disk Utility & Toast) say that they are 2 or 3 times bigger and thus need a DVD.
Why is that ?
Is the disk image compressed?
If so, then the .dmg file is probably going to be about 2 or 3 times smaller than the data it contains. When you mount the image, you can do Get Info on the mounted volume to see the volume size, which is how much space you will need on actual media.
Even an uncompressed image file may be smaller than the volume it contains as the image file may not include unallocated space.
Different file systems and disk formats have different block sizes, which all files are rounded up to in size. If you have a lot of small files on one file system, and try to move them all to a different file system or media format with much larger block sizes, the total space required can balloon.