> I had one open connection that was left open for few hours and it kept
> the oldest fields so low.

May be this connection (actually - transaction inside it) was the
reason why database grown?..

> Oldest transaction 96722
> Oldest active 96723
> Oldest snapshot 96723
> Next transaction 96724

Perfect statistic. If it always looks like this, sweep will never occur.
Perhaps (because of CS) there is no queries which would collect garbage.
I would return automatic sweep interval to default value and run
manual sweep nightly. Or just occasional queries like SELECT COUNT(*)
FROM TABLE to collect garbage. Or backup.

SY, SD.