For road cars yes, a bit is because of safety. For the US its mostly because of the one law about light trucks being work-vehicles and so they didnt get taxed the same or something.

I believe a large part of the size is for aero reasons. For example, the wheelbase is extended by something like 50cm if I recall correctly. Bigger car equals more surface area to fit aero devices on. I think it was the teams pushing for bigger cars to make them go faster.