If Virtual PC 2007 is about 30MB, how come VirtualBox is about 70MB, about double the size?

Quite often, size is irrelevant when it comes to executables or installers. They can be packaged differently, have higher compression or just offer more things. In my opinion, VirtualBox is more fully featured than Virtual PC.

"Maybe Microsoft has included parts of code required for the virtualization in the Windows kernel or in system DLLs.

That would explain why VirtualBox is bigger, as they have to deliver all the code necessary for the virtualization, the GUI, the tools, ... while only a part of it is included in the VirtualPC setup."

VirtualBox is by far the better option IMO. It supports USB, and the DirectX/OpenGL acceleration passthrough.

Inside the Vbox package of 70Mb, you will find the Guest Additions which account for 31 Mb. Would that account for the size?