My understanding is the following...
When you do cardio, you conciously know how long and how intense you plan to exercise. Your body has no way of knowing this.
So when you exercise at the lower intensity, your body reacts by throwing fuel on the fire so to speak. It does so from fuel that's already in the bloodstream, followed by fuel from stored energy (fat), followed by fuel taken by metabolising muscle mass. At the lower intensity, there isn't a huge need for energy, so it is fairly content by using a proportion that uses a lot more fat than muscle.
When you go with the high intensity cardio, your body will think it needs all the fuel it can get and throws anything on the fire that will burn. The ratio of muscle to fat increases. Remember that your body has no idea how long you plan to exercise, it just knows that it needs energy NOW!
What that adds up to is the following:
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[*]High intensity cardio burns a higher ratio of muscle to fat than low intensity cardio.
[*]High intensity cardio burns more calories than low intensity cardio in the same amount of time.
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The more calories you burn, the more weight you lose. The source of the weight depends on the way you burn your calories. If you exercise for 20 minutes at high intensity, you'll burn more calories than exercising for 20 minutes at low intensity. But a bigger chunk of the calories burned will come from muscle mass instead of fat. That's why it is said you need to load with protein afterwards; to replenish what was burned. The only problem with that is that you are now adding calories back into your body which could potentially turn back in to fat.
To burn the same amount of calories with low intensity, you need to do it for longer periods. The trade off is that you have more calories being burned from fat than muscle. It just takes longer to acheive the same amount of calorie loss.
This is just what I've heard on the subject. I'm sure it will be proven and disproven many times in the following years. Personally, when I am cutting, I like to save as much energy as I can for lifting which means I only do the low-to-mid intensity cardio and save my strength for when it matters.