my calorie burn reduced

Yes you will burn less the smaller you get but proportionally it will be the same. It's just one of those things. Just to add how are you working out that burn? If you're doing it via heart rate monitor it is unlikely to be accurate because HRMs are only set to be used for steady state cardio which fitness videos aren't

It takes less energy to move a smaller body around, it's just physics.

Exercise for fitness, not the number of calories you burn.

There is more than one question/issue here:

1. Accuracy of your workout calorie estimates. If you are using an HRM, decreased weight will lead to decreased calorie display. As your fitness level improves, you will experience a lower heart rate when doing the same exercise intensity, and that will also cause the HRM to display a lower calorie number. As one's fitness level increases, one needs to make the adjustment in their HRM settings to account for this, but that is not possible with all models. So, basically, your HRM has some definite accuracy issues when your fitness level changes over time.

2. For someone who is using a cardio machine that allows measurable increases in workload, or who is doing an activity (e.g. running/walking) for which changes in workload are easier to measure, the response to increased fitness is straightforward--you just increase the workload, walk/run faster, etc. Since calories burned is equal to Body Wt x Exercise Workload (in METs), working at a higher workload will offset the decreased calorie burn that comes from losing weight. (e.g. a 100kg person working at 10 METs burns 1000 cals/hour; a 90kg person working at 11 METs burns 990).

However, workload intensity is a lot harder to measure and regulate when doing a group class or DVD. Even if fitness level improves, the ability to work at a higher workload might be limited by the person's coordination, physical movement ability, or class structure/choreography. In that case, a person losing weight might not be able to offset the weight loss by working harder, and thus might be getting an inferior workout both for fitness AND for calorie burning.

If you are doing the same class, you can possibly use your average HR per workout as a rough gauge of progress. If your avg HR in the beginning was 150, for example, and now it is 120, then you need to push yourself harder to get it back into the 140-150 range, and if that is not possible, it might be time to choose another program.

While I don't care for the tone of a lot of the comments, I agree in general with the idea that you don't want to focus solely on calorie burn when planning your workouts.