In the diagram, as you go deeper, the load spreads which means it is spread over more and more soil particles and each particle supports less of the load. The load per soil particle drops off quickly. But as you go deeper, the effect of the other wheels starts to become felt as the load 'cones' start to overlap, and the load per soil particle increases. However the load spreading reduces the load more quickly than the overlapping cones effect increases the load.
Better to have thicker concrete at the top where there isn't yet much load spreading, because the strong concrete particles can take all the load without squashing.