Formula for figuring calories burned?

yes a person weighing 300 will burn more than a person weighing 150....but I do not know of a formula to figure out how much.
BUT I might suggest you get a heart rate monitor and you can program your stats in it, and it will tell you how many you burn, they are pretty accurate.

I am not an expert, but I think it has more to do with the duration and intensity of your work out. A calorie on a larger person is the same as a calorie on a smaller person. To eliminate that calorie, there has to be a certain amount of exertion for a certain time and intensity. Less intensity, more time. More intensity, less time. The only thing that I think could be different between two people is metabolic activity but then I think a larger person would have a slower metabolism than a smaller person which would tend to suggest the opposite of your question.

But I can see this as a scenario. Say I am the 300 lb person and you are the 150 lb person and we both run on a treadmill for 30 minutes with all the same settings and each reach 3.0 miles. You being the 150 lb person would likely have an easier time completing this workout and therefore the intensity for you is less. I being the 300 lb person am doing the same activity with an extra 150 pounds so my intensity could be significantly higher. In this case I can see where the 300 lb person would burn more calories. Not sure how to quantify it though.