exFAT (Extended File Allocation Table) is not a formerly obese person, but a filesystem created by Microsoft that is designed for flash memory devices. It supports higher limits to sizes of files and of the entire partition than earlier FAT filesystems such as FAT32. It is a proprietary Microsoft system with no publicly-released spec, though efforts to reverse-engineer it for an open-source project (e.g., fuse-exFAT) have proceeded. Such projects could, however, run into legal problems since there are patents pending on the system.
While it is a Microsoft system, exFAT is designed to be compatible with Mac OS X 10.6.5 as well as Windows XP and up.