It's not the weight of the pillion passenger that is the problem. It's the weight differential between the driver and the pillion, and the weight distribution load on the front and rear, especially the rear. Get a skinny pillion that likes to sit as far back as possible on the seat and they become the same as an overweight pillion - i.e a huge MF sail and disproportionate counterweight. It doesn't matter if the driver is overweight as his/her weight isn't way behind the frame pivot point or swingarm. But a 60kg jockey and a 100kg passenger is a difficult bike to ride. Especially your typical Japanese scooter where the chassis/frame is about as bendy and flexible as a rubber band. Even calling it a "frame" is being generous - those things bend and flex sideways horribly even without an overweight passenger. Hats off to the taxi-driver for pointing out the safety issue, and no points to the overweight passenger who believes his/her dignity can deny physics.
