Old Akro, the thicknesses are just total pavement thickness – in the case of the Cessna, it is all gravel and assumes that the natural ground is not too weak or in a bog. In the case of the A380, it is 125mm hot mix asphalt plus 250mm cement stabilised crushed rock plus layers of crushed rock and gravel. The life of the Cessna runway is a bit limited because gravel erodes by up to 25mm a year (wind, sun, rain, abrasion etc), so some people would put an extra 25-50mm down at the start to allow for a couple of years of erosion.
If you wanted to put an engineered surface (bitumen or concrete) on top of the gravel from the start for the Cessna, then it is much less tolerant of soil movement and pavement deflection and you would make the gravel a bit thicker.
The tyre pressures and weights of the Cessna are almost too small to count in pavement structural terms.