pg_subtrans directory grows over 1 GB, is there a way to control it?

I am using PostgreSQL database in a location recognition application, the files under postgresql/cluster is growing and over a few days it become very large as below. 
    /var/opt/postgresql/cluster/data  166015 Kilobytes  (the actual database data)
    /var/opt/postgresql/cluster/sys/pg_clog 74188 Kilobytes  (transaction commit status data)
    /var/opt/postgresql/cluster/sys/pg_subtrans   1187032 Kilobytes (transaction status data)

The pg_clog and pg_subtrans can grow when database contains more data and more queries come in. But these figures looks rather large to me because the queries goes in the database are mainly read operation rather than write one. I got a few question to ask here:
     Is there a way to control the size of pg_clog and pg_subtrans? 
     Is the file size still be normal for the database of this size (166MB for cluster/data)?
     Does the larger pg_subtrans indicate some bad transaction happens in the database?

You expertise is very appreciated.

Thanks

Dennis



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