Sorry about not getting back to you sooner...
max_blocksize determines the size of the block examined for changes during the diff process. For files < 1MB the blocksize is a constant of 512. For files over 1MB the size is given by:
file_blocksize = int((file_len / (2000 * 512)) * 512)
return min(file_blocksize, globals.max_blocksize)
where globals.max_blocksize is defaulted to 2048. If you specify a larger max_blocksize, your difftar files will be larger, but your sigtar files will be smaller. If you specify a smaller max_blocksize, the reverse occurs. The --max-blocksize option should be in multiples of 512.