HRM and Calories Burned

Hey everyone. I am new to the message board part of MFP. Not sure how I didn't know it existed. I have a question I am hoping someone can help me with.

My workout buddy and I bought Polar FT4's a few weeks ago. We do the same workouts when we are together and she is burning 200+ more calories than me almost everytime. My heart rate stays within my range for 75-80% of my workouts, her monitor shows her rate is always higher than her range and she is usually only in range for about 25% of the workouts. She is 5'4 and 246lbs. I am 5'8 and 270. I workout hard, I don't just pretend. Tonight I tried to get my heart rate above my range and no matter how hard I worked on the elliptical my heart rate never got above 158. My max is 162. I don' t understand how she can burn so much than me when alot of times she has to stop for breaks and I keep working. I thought being in your target heart rate zone was good, but it seems like she is burning way more being above her zone. Im confused. Should I be trying to get above my target zone to burn more calories? I always thought the more you weigh the more you burn, but that doesn't seem to be the case for me. I usually burn anywhere for 400-700 calories working out, depending on what I am doing that day. It seems like I should be buring more a day, but none of the information I have found on the internet is very clear about it.

Thanks in advance.
Diane


Thanks in advance.
Diane