How do increase calories burned during cardio

Heart rate is a relatively accurate way to measure calories for steady-state cardio. As your effort increases, your muscles call for more oxygen and other nutrients. The body responds to this increased demand by increasing blood flow to deliver more of them faster. You aren't actually burning that many more calories from the actual increase in heart rate - the heart is not a terribly huge muscle - but the heart is an indicator that your body is probably calling for oxygen.

There are other things that can increase heart rate. Anxiety, anger, that attractive member of the opposite sex walking by, etc. Few of those things actually increase calorie burn, so wearing a heart rate monitor all day to try and estimate calorie burn is pretty useless.

But when you are elevating your heart rate specifically for the purpose of exercise, it's a relatively consistent ratio. And it doesn't matter how fit or healthy you are - if your heart rate is at x for a given weight, height, age, etc - you are probably burning y calories within a reasonable margin of error. As you get more fit, you must maintain the same effort (and therefore increased athletic performance) to have the same calorie burn.

I weigh over 200 pounds. At my age, my "max cardio" (85% VO2) range of 150BPM, which I can maintain easily for an hour, I can burn 960 calories an hour doing any cardio exercise which supports reaching and maintaining that heart rate steady-state. Treadmill, elliptical, running in place, etc.

For my wife, who weighs closer to 160 pounds, maintaining 85% VO2 for an hour is a somewhat less promising 580 calories.

Source: http://www.shapesense.com/fitness-exercise/calculators/heart-rate-based-calorie-burn-calculator.aspx

At the end of the day, it's all estimates based on averages, but heart rate monitors are the best tool I've ever heard of for steady-state exercise. As far as true daily burn, it's hard to find something that can be anywhere near accurate. FitBit goes mostly based on motion, but it can't tell how much of your body is moving. Evolution failed to equip us with an honest fuel-rate meter.

Most I can burn is around 600-650 calories in an hour, given I am only 125/126 Ibs and that is using the elliptical or running/jogging on the treadmill with my heartrate remaining between 80 and 90% of max. Cycling with lots of hills burns me around 480 an hour, and that is all according to my polar ft4 hrm. The cycling was pretty close to the mfp estimates anyway.