It's all a matter of how big of an area you spread the weight over. The pressure exerted on the ground is more-less directly proportional to the PSI in the tire. The difference is made up by appying lower pressure to more area. Since we are dealing with a moving object, the pressure on the ground is lower, but it is exerted for a longer period of time before the tire has rolled on.  Belts lengthen the period even longer. 

Going wider increases the surface area which is compacted.  Going to higher pressure increases the depth of the compaction.  Going to a longer contact patch actually reduces the compaction profile because you are lenghting time as the fourth diminsion.