sparse image, the UDIF variant that has variable size, UDSP.plain disk image, known as a UDIF read/write image, abbreviated to UDRW.The most common types of Disk Image you’re likely to come across are:

Create a 10 GB sparse image or bundle, put 1 GB inside it, and it will be little more than 1 GB in size, as it doesn’t store any empty space. dmg file will be 10 GB in size, even though most of it is empty. Create a normal 10 GB disk image to contain 1 GB of files, and that.

There are two main types of Disk Image: regular images, which include not only the data stored in them but also all free space, and sparse images/bundles which try to store just the data without any free space. This article explains what’s on offer in macOS, with particular emphasis on sparse bundles. What’s so complicated about Disk Images are their many variants and options. They’re commonly used to distribute software, to protect sensitive documents using encryption, and to provide a virtual file system for Time Machine backups on other host systems with networked storage. Generally, a Disk Image is just a file or bundle (a specially structured folder posing as a file) which can be ‘attached’ by macOS and function like a whole disk, with volumes, folders and more. Disk Images may be old hat, but we’re using them more often and they’re far from straightforward.

Mac disk image size