Speaking as someone who drives a big heavy truck that doesn't have the miracle invention known as power brakes (they were an option in 1970), when stopped at a red light for a long time my foot and leg gets tired and I creep forward for a bit to readjust its position on the brake pedal or in the case of really really long stupid stoplights to switch feet. For the heavier a vehicle is the more force one has to apply on the pedal.