No, it would most likely be screen and processor technology that is holding up a slimmer full sized iPad.
The main component that dictates the size of the iPad (besides the screen itself of course) is the battery. The extra backlight required by the retina display is what caused the iPad's battery to grow from the iPad 2 to the 3rd gen.
To slim the full sized iPad back down will require a new screen technology, like Sharp's new IGZO displays, that require less backlight power at high pixel densities.