Step-by-step explanation:
the wheel has a lower surface area that is in contact with the ground yet, is narrowed to a large surface area (ground). when a load is shifted towards the wheel, the weight of the load is narrowed and focused on the low surface area.
this decreases the pressure exerted by the weight as the surface area is narrowed down yet is conveyed to a large surface area (the ground) despite the force being constant.
hope this helps. :)