How can one determine the TRUE size of an assembly?

For example, I have an assembly file where windows explorer tells me its 20 megs in size.

That may be true for the assembly file, but when you open the file SW loads (in this case) thousands of other parts and features.

when fully loaded, I know this file is far larger than 20 megs. So, how do I find how much memory this thing is really eating up?

I considered looking at memory levels in task manager, but thats not going to be quite right do to paging.

This same question also applies to drawings, which may look small, but loads tons of stuff when opened.

Thanks