There are dud dmg files on the network or people have made a poor effort at creating a dmg file with incomplete data. So normally, the larger the dmg file the more likely it will be useful. Some dmg files are replicas of the disk in which is came from, some have been created from a disk but are missing some files, yet some have been custom made & missing some important support files. But then, some dmg files are not from disks but how they were offered for download from the origin site.

Another issue with dmg files is if they were put together on someone's system which has directory problems. Thus other people can't open them.

Other types of disk images used for mac are 3rd party or from windows ... such as Toast images, or the windows disk image formats such as BIN/.CUE & iso etc.