At the moment OGS has approximately 6350 groups.
About 10% have 20 or more members, but about 20% have only one member or no members at all. Approximately 70% are small groups (between 2 and 20 members).
I wouldn’t be surprised if a lot of groups are not active, haven’t been active for some time or have been inactive forever. I don’t know if it is possible and/or allowed, but how about (a) asking group admins if they want to maintain their group and (b) if not interested in maintaining their group / no reply: delete these groups?