Sparse image

A sparse image is a type of disk image file used on macOS that grows in size as the user adds data to the image, taking up only as much disk space as stored in it. Encrypted sparse image files are used to secure a user's home directory by the FileVault feature in Mac OS X Snow Leopard and earlier. Sparse images can be created using Disk Utility.
Unlike a full image file (), which takes up as much actual space as the real disk it represents (regardless of the amount of unused space), a sparse image file () takes up only as much actual disk space as the data contained within, up to a maximum of the capacity assigned during creation.
Limitations.
Two limitations are therefore worth noting regarding the use of this image file format:
Sparse bundle disk images.
Mac OS X Leopard introduced the concept of the "sparse bundle". Instead of a single big file, a sparse bundle is a bundle (directory) containing a number of files called "bands", each on the order of 8 MB in size. This means even though to the end user the sparse bundle appears as a single file, it is composed of smaller files. As of Mac OS X 10.8, the bands are 8 MiB () each. When the content of the image changes, one or more band files is changed, created, or deleted. This allows easier shrinking of the amount of space used on the underlying filesystem when the a large amount of data is deleted inside the disk image, as long as the code supporting the filesystem inside the image notifies the kernel of the "releasing" of blocks, as happens in filesystems that support SSD "trim" operations to be triggered automatically for de-allocated filesystem blocks.
Sparse bundle advantages over non-diskimage backup for Time Machine: