Default cluster size for NTFS, FAT, and exFAT
support.microsoft.com › en-us › topicFormatting a volume from Windows Explorer when the Allocation Unit box in the Format dialog box lists Default Allocation Size. By default, the maximum cluster size for NTFS under Windows NT 4.0 and later versions of Windows is 4 kilobytes (KB). This is because NTFS file compression is not possible on drives that have a larger cluster size.