While people who live at altitude ultimately come to utilize oxygen better than those who live at sea level (though the steepest performance advantage is said to be for those who train low and sleep high), this does not necessarily translate into a workout that burns more calories.
"Your ventilation increases," says Armstrong, "but this perception of increased effort doesn't necessarily mean you're expending more energy. The only increase in energy output would be due to the increased work performed by the rib cage and the muscles involved in breathing. Other than that, you may have a breathless feeling but that doesn't mean you are expending more energy. Additional energy output would be a function of workload, but other than the increased work spent breathing, at a higher altitude you still are doing the same amount of work."