Hash table size in MB; must be a multiple of 16.
A larger ratio of index size to storage size means smaller blocks of duplicate content are recognized.
If you have 1TB of data, a 4GB hash table (which is to say, a value of 4096) will permit 4KB extents (the smallest possible size) to be recognized, whereas a value of 1024 -- creating a 1GB hash table -- will recognize only aligned duplicate blocks of 16KB.