SegmentInfoPerCommit continues to list updates files even if they are unneeded anymore. For example, if you update the values of documents of field 'f', it creates a gen'd .fnm (FieldInfos) file. If you commit/reopen and update the field again (maybe now a different set of documents), it creates another gen'd .fnm, but continues to list both gens, even though only the latest one is needed.
To solve this, SIPC would need to know then dvGen of each FieldInfo, so that it can correctly list only the updates files that are truly needed. I'll work on a testcase and fix.