Well... you actually are probably burning higher number of calories because your heart is working harder carrying extra weight. i know for me, when I was 290, I burned a heck of a lot more calories than I did at say, 250, doing the exact same workout. The better shape you get in, the harder you have to work to burn calories. That being said, if you really want to know exactly what you're buring, I would recommend investing in a heart rate monitor. I love mine!!