Not quite, mdoo. When crude oil is cracked, different hydrocarbon products come out. Some of the lighter products are suitable for petrol, whilst diesel is a heavier product. The proportion of what the refiner gets depends on the quality and source of the crude, and is (slightly) adjustable by the refinery varying the cracking parameters.
After that it is supply versus demand for the produced product. Over the last few years demand for industrial heavy heating oil and motor diesel (which are similar products) has increased more than that of petrol (relatively). So the price goes up more.
Bitumen is another byproduct of cracking - a very heavy product. Its price is low because the refiners cannot stop it being produced (they can try and limit it). If someone found a creative way of using bitumen as a fuel, the price would soon rocket.