I just noticed this problem on my site too. over 65,000 rows in this table!!!!!
Data gets logged in this table every time there is a user logout. The problem is this data only gets removed if the "remote login" option is turned on. The query that clears this stuff out doesn't trigger when remote login is off. This seems like a small oversight in code but has huge potential for bloat on busy sites/networks.
Most of these logged rows in the domain_mapping_logins table are supposed to be deleted if they're older than 2 MINUTES by the plugin, but again, the query never runs if remote login is off.
Crazy town!
I'm manually disabling this query that adds the data to the DB because it appears the data only seems to be relevant if remote login is being used (comment out line 680 in the most recent version of the plugin).
The long term solution IMO would be to have the plugin updated to not log this info at all if remote login is off. I may not understand the intent of this info though.