One of the big reasons for the stupid size of parking lots around stores has to do with zoning laws as well. I don't remember the numbers and it's gotta vary by district, but it's tied to a certain percentage of the max occupancy of the building.

Because of the car-focused infrastructure that puts everything else last, we've created a self-defeating system that forces people to drive everywhere, thus justifying the massive parking lots that prevent people from using other modes of transport in the first place.