rndc flush

Hello.
I'm running Bind 9.2.2.
My understanding is that executing "rndc flush" should clear up the
cache that Bind has been building up with time.
When I execute rndc flush, the server's CPU goes to 100% for a couple
of minutes, but the memory that the process named takes up does not
come down.
After named loads in my server, named's footprint is aprox. 15MB.
After a few days of operation, it goes to aprox. 120MB. Shouldn't the
amount of memory the process takes come down to aprox. 15MB after
"rndc flush"?

Thanks for any insight you may give me.

Arie Kachler