It was discovered the database had grown from a previous explosion of the events table, but the mah2 didn't shrink when those events were purged. H2 does some compaction at each shutdown, but it's not reliable that it will give the smallest DB for the existing configuration. Like many database servers, it will hold onto the disk space it is given, presuming databases tend to get larger.
In this albeit somewhat atypical instance, creating a backup and then restoring from the backup (after starting Mango on a clean DB) shrank the database from 3.7GB to about 5MB