Our case is the opposite, we had almost 50GB backup files under R77.30, probably because of many historical database revisions we had kept in each CMA. Under R80.10 the backup is about 22GB, because older revisions got lost during the migration. During the last 3-4 months the backup has grown 1GB, which is not terrible and usually it takes approximately 1h20-1h30 to create the backup file. Restoration time is about 1 hour in a vmware lab, but solr rebuild takes additional time indeed.
From time to time it happens that pg_dump processes will stay in the process table even after the backup file has been created, sometimes dbedit locks are not removed etc, I would say that older pre-R80 backup was more bullet proof than the current one. No positive comments about R80.10, no quick crossdomain search, the SmartConsole is buggy, it has short freeze moments all the time which can even observed on CP demo streams, full text search not working properly etc.