bigger bikes are heavier bikes and more difficult to maneuver, both at slow speeds and with turning and cornering. they are also generally a lot faster than the smaller cruisers because of their increased displacement and subsequent HP, and managing the increased speed and acceleration, along with the handling of a heavier ride, are skills that are better suited to a rider that has some experience. therefore, a lighter, smaller, and less quick bike are better suited for a "beginner" because it's easier to operate versus the bigger bikes. you wouldn't want your newly licensed teenaged kid to start out behind the wheel of the new Dodge Demon.
also, as mentioned before, what constitutes a big bike today was not available 20-30 years ago. today's medium or smaller bikes used to be the big bikes. so you have a generation of bikers who rode those smaller sized bikes and want to graduate to something more powerful as new iterations keep becoming available.