Heavier people do burn more kilojoules per minute than their lighter counterparts when exercising at the same intensity simply because they are carrying more weight.
To get an idea of how much visit the Daily Calorie Needs Calculator in the tools section of this site ( http://www.weightloss.com.au/Tools/BMRCalc.htm
) and just change the weight variable and repress the calculate button.
The differences between the calories per day figures will be a combination of the extra calories needed to maintain a heavier body and the extra calories burned during exercise/activity.
I have some specific figures in the office about weight and calories burned during exercise that I'll post here for you shortly.