The byte by byte view makes the failure mode people run into easier to explain. VACUUM can never remove a tuple whose xmax is newer than the oldest snapshot still open, so one forgotten idle transaction or an abandoned replication slot pins that horizon and every autovacuum pass does a full scan while reclaiming almost nothing. When a table keeps bloating even though autovacuum looks healthy, backend_xmin in pg_stat_activity and xmin in pg_replication_slots are the first two places to check.