Those figures are likely to be constantly changing as a) the ashServ.exe is the main scanning engine of avast, b) the web shield as you brows web pages that to will change, but they don't seem excessive.
The other point of consideration is where you are getting the size values from as the Task Manager isn't too accurate and within that there are several different size calculations, the VM Size has in the past been said to give a more relevant indication. See image of my figures as I'm posting this.
However, there are times when that is higher, but you can get an idea that there are great variations in how memory use is reported. In the greater scheme of things memory is there to be used or you would have a slower scan and more swapping out to the hdd both of which slow your system.
avast is by comparison to many other AVs relatively light on resources.
