Large files in main/base

Hello everyone,

We had a problem with one of our servers and had noticed that the
postgres/8.3/main folder had become quite large (>650mb).
PostgreSQL confirmed this via a query but when I queried the size of the
tables I was barely reaching 3mb.

After looking around, I found one file with about 580 mb. From the
contents I have the feeling it is some kind of changelog or similar.

My Questions:
What is it?
How can I reduce it?
How can I prevent it from reaching that size again?

Thanks

Frank

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