Mac OS X Leopard introduced the concept of the sparse bundle.[5] Instead of a single big file, [b]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 (8 × 10242 byte) each[/b]. When the content of the image changes, one or more band files is changed, created, or deleted. This allows backup software (such as Time Machine) to operate more efficiently. A tool such as rsync may be used to keep one or more disk images consistent across various systems.[6]
https://en.m.wikipedia.org/wiki/Sparse_image
