Why pg_toast table not get auto vacuumed?

Hi,

PostgreSQL version: 9.1.16. Linux: RHEL6.

After a heavy traffic run, a huge pg_toast table is seen. Its size is ~3G.
There also exist other pg_toast tables in a much smaller size, for example,
~100M. The problem is that that huge pg_toast table never gets auto vacuumed
while other small pg_toast tables do get auto vacuumed, as shown in pg logs.
If running traffic again, the huge pg_toast table grows again.

We have another system with the same setup but we only run light traffic on
it. The pg_toast table from the same parent table also exists. But this
pg_toast table only has a size ~50M and it gets auto vacuumed from time to
time as shown in pg_logs.

Why does not the huge pg_toast table in the first case get auto vacuumed?
Are there anything we can do?

Thanks,
JL

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