Do you have a baseline of what memory utilization used to be? did it become high after you confgured the tunnels, my opinion is this could be normal for a 16 megs of RAM firewall, unless you start seeing pix stoping traffic or crashes then I would start looking into solutions,but! I would definatly recommend to boosting your memory to 32 MB.

Read this link to give you an idea of things to look for, if none applies I would consider placing more memory in your firewall.

http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a008009491c.shtml