I suggest the following changes:

* `subset_size` - drop because, as you say parent agg or query hits should provide that stat
* `superset_size` - change to `superset_doc_count` ("superset" not "index" to allow for abstractions like aliases/filtered sets)
* `sub/superset_doc_freq` - keep, "doc_freq" is consistent with the meaning in "IDF" and the 'min/max_doc_freq' in MoreLikeThis.