Mine is currently at Men Usage of 24,156K, Peak Usage of 38,664 with a VM size of 100,00K by all accounts (avast team) the VM Size is a more accurate guide.
The amount of usage will fluctuate in use and I wouldn't be unduly concerned unless the CPU % is also elevated and not 00% when idle ?
Have you recently downloaded anything ?
You could Terminate the web shield and start it again, I just did and now my figures are, Mem Usage 2,288K, Peak Usage of 14,336K and VM Size of 8,956K