Referencing this page: https://support.microsoft.com/en-us/kb/140365
NTFS doesn't go over 4KB cluster size until the volume goes over 16TB, and FAT32 maxes out at 16KB in the 16GB–32GB volume range on modern versions of Windows.
exFAT however only defaults to 4KB in the 7MB–256MB volume range. After that it jumps to 32KB in the 256MB–32GB range, and 128KB beyond that.
Why is that? A relatively high cluster size seems wasteful, especially in a format designed for smaller external devices, like flash drives. Doubly so if you're formatting flash memory, because as I understand it, the main benefit to larger cluster size is faster IO due to less fragmentation and less overall clusters to read. Correct me if I'm wrong, but flash memory is much, much less prone to slowdowns caused by fragmentation. So why make the cluster size so high?