My elliptical has a heart rate monitor strap which you wear. Doing a similar routine (or even harder!) than using a gym elliptical, my elliptical reports about half the calories that those in the gym would normally report.
An example would be heart pounding elliptical workout with intervals for 25-30 at home usually nets me around 200-250 calories. If I did that same workout at the gym, the gym machines would normally say around 500 calories.
You can't out-exercise poor eating habits.