While using OWB for a while I notice that a lot of RAM becomes unavailable, from opening TABs, and eventually closing them all, but one.

So starting off with just about 900MB of RAM it gets to a point that there is basically no RAM left, even though OWB is the only application that has been opened, and at that point there is only one window/TAB left open, but I can't find a way to free back up the rest of the RAM in my Mac Mini that should be available.

Is there some utility that can release this unavailable RAM?

I tried 'avail flush' but that barely releases any of the RAM.

As soon as I close OWB the RAM frees itself back up, and then I have just over 900MB of RAM available again, so somewhere within the program it isn't releasing what it's not actually apparently using - is this a known bug?