You are right, there should be the same amount of rendering in FF 6, not more. What seems to be going on: in FF 5 the built-in images are displayed "pixel doubled", that is, 1200x1200 (at least on a Retina display), and the Actual Pixels button is greyed out. The rendering is still 600x600.
So one way is to duplicate how the images were displayed in FF 5, "pixel doubled", with no additional rendering.
Another way is to keep FF 6 the way it is now, but keep the current "zooming" state between presets (and between program launches if possible). With this method you can use the Actual Pixels button "correctly".
My 2 cents, 3 with inflation.
Math meets art meets psychedelia.