Bro, I did mention engine would have to overcome more friction, which in turn means more torque (or rotational energy) required.
A small experiment would be to put 10 kg in a trolly when you go shopping Hyper Star or any mart, and roll it down some distance. Next, double the weight and repeat the roll thing. Now again put 10 kilos of stuff in the trolley, this time doubling the wheel base, and push it through the same distance. (I am not suggesting you actually change the cart's wheels,
but) intuitively speaking, increasing wheel base wouldn't require you to add a significant amount of force to push the cart. However you would feel the need to push with more force when you double the load without altering the wheel base.
Based on this argument, a pillion rider would cause more trouble to engine than just increasing the wheel base. Engine would be happy to carry the same load despite the rear tyre offering a slightly increased friction (or stress, so to speak).
PS: I am happy to see a medical professional having interest in Physics (mechanics, in particular) 