First i would like to say that both riding and weight lifting are wonderful and when done right can compliment each other nicely. In terms of energy expenditure, they are not created equal. I think the confusion is in the difference between riding and racing. People often come to me and say that it is not that hard to ride a motor cycle. I usually agree but then state that racing is an entirely different demon. Think about what your whole body is doing when you race, or run your practice laps. there is not one part that isn't in some way active. The same cannot be said for traditional weight lifting. While you are on the track you are standing and absorbing multiple G-forces just going through turns, taking off jumps, landing. Motocross is extremely ploymetric. It requires multiple training adaptations: Power, to be able to absorb landings acceleration bumps. endurance, to stand for 15-30 minutes of continuous strenuous activity. Strength to correct your bike and/or body when thinks get out of control.
As a strength coach and trainer I have always wondered how many calories could be burnt while riding. I wore a Polar F6 heart rate monitor when my brother and I competed in our first cross country team race. It was 70 miles with each lap being 10miles. We switched off every lap and I rode 4 out of the 7 It took us just under 2 hours to complete. I estimated my time to be about 1:05-1:20 total time and the other ~45min I sat waiting for my bro to come around. At the end of the race My heart rate monitor read over 2,000+ calories. I also noticed in my text books from college that it is given a low expediture rate. I am not sure what they measured to get those findings but it wasn't racing.
FYI: My bro and I won the race by over 6 minutes
Hopefully this was helpful.......