Keep in mind that small engines are governed RPM-wise so as long as a given engine has enough power to drive the thing, ground speed will remain close to the same. What about the pully diameter of each engine? A larger pully will result in a higher ground speed as it will be spinning the trans faster (assuming this model is pully driven).
You actually found the most probable reason; Larger rear tires will make it faster.
You actually found the most probable reason; Larger rear tires will make it faster.