Harder to get the heart rate where you want it. You lose some of the resting burn and some of the active burn when you lose weight. You have to do more just to stay even. That’s part of the trickiness of losing weight. Once you’re in maintenance, at least things stay a little more steady state.You will also find that the more fit you become the more you have to workout to achieve the same amount of calorie burn. Our bodies learn to become super efficient so the amount of calories burned greatly depends on many different factors. That's why calorie calculation is so difficult and all of them are nothing more than an educated guess