following is current doc: (c)XSIBackup Block Size
--block-size[=1M(default)|10M|20M|50M]: (optional) this is the block size that will be used to deduplicate data when using the --backup action. In case of replicas a fixed block size of 1M will be used. You can choose between: 1, 10, 20, or 50 megabyte block sizes when performing a --backup action. 10M is the default --block-size.
Note how that at beginning default is 1 M, and at the end it's 10M.