From: Performance and workload studies

The last block of a file is normally only partially occupied, and so
as block sizes are increased so too will the the amount of wasted disk
space.

The following historical values for the design of the BSD FFS are
given in `Design and implementation of the 4.3BSD Unix operating
system':

fragment size   overhead
   (bytes)        (%)
      512         4.2
     1024         9.1
     2048        19.7
     4096        42.9

Files have clearly gotten larger since then; I obtained the following
results:

fragment size   overhead
   (bytes)        (%)
      128         0.3
      256         0.6
      512         1.1
     1024         2.5
     2048         5.4
     4096        12.3
     8192        27.8
    16384        61.2

By default the BSD FFS typically uses a 1k fragment size.  Perhaps
this size is no longer optimal and should be increased.

(The FFS block size is constrained to be no more than 8 times the
 fragment size.  Clustering is a good way to improve throughput for
 FFS based file systems, but it doesn't do very much to reduce the not
 insignificant FFS computational overhead.)

It is interesting to note that even though most files are less than 2K
in size, having a 2K block size wastes very little space, because disk
space consumption is so totally dominated by large files.