Thanks for your post!
You've probably found this from your web searches, but what I've found is that agcore.dll is the Silverlight core dll used mostly for drawing, and npctrl.dll is the plugin used to host the Silverlight runtime in a browser and interact with it.
Have you tried looking at the class list to see which classes are using the most unmanaged memory? That will hopefully shed light on the high usage of the Silverlight libraries.
Red Gate Software Ltd.