I think that the decision to tie the hash or not depends not upon the size of the file that is being read but more upon what percentage of the file being read has meaningful entries.

If the majority of the entries in the file being read are just junk that can be ignored, then the state-hash can probably be maintained in memory w/o tying.

If the data source is very rich, though, then it would we wise to tie the state-hash to another file and manage the entries.