I was thinking a bit about the physics of lifting weights. Just off the top of my head:
I will do a 25-rep sequence in about 5 minutes with 66kg. (145lb). I estimate the bar moves about a half meter, and if I'm lifting with "snap" then the lift takes about half a second.
Power is (force * distance) / time = (66kg* 9.81m/s^2)*0.5m / .5sec = 647 watts
Wow! That means I'm almost as strong as a horse! (one hp= 746W)... but only for half a second.
In the 5-minute sequence this happens 25 times. The total energy expended is 25*(66kg)(9.81m/s^2)(.5m) = 8093 joules. What's a joule? Interestingly those convert straight to calories.
7970 J = 2.0 C
WHAAAT? only two calories? That's a tic-tac! Well, there are a few other things going on here. First off, it takes energy to bring the bar back to the chest with control. Maybe, half the force for a full second each time... that would add 25% to the watts and 50% to the joules. So, three calories.
To me, this points out something very interesting: counting calories out is about as useful as counting calories in... from an evolutionary fitness perspective. You will "burn" a lot of calories treading a mill for 40 minutes. You will probably "burn" as many calories racking your weights as you did in the actual exercise.
So what's the point? What I did do for two calories that I didn't do for 200 calories of treadmill walking is build a structure in my body that requires fuel all the time, whether I'm in the gym or not, and will burn that fuel from the protein and fat I eat or store in my body.
Think of it another way: suppose you have a 2500 calorie/day diet. A hard-core treadmill session might "burn" 400 calories. Where do the rest go? If your weight is stable, you have to be using the vast majority - 2100 calories - somewhere else. You do it walking, sitting, standing, sleeping.
Think of how much energy it takes to maintain body temperature! What if you had your body weight in water? For me that's about 66kg, so 66 liters of water... so, imagine 3 cases of Perrier. (Hey, I'm French.) How much energy would it take to maintain that much water at 37C / 98F? A heckofalot more than it takes to lift a weight a few times, beleive me.
To summarize: the workout is not to burn the calories. The workout is to build the engine that burns the calories when you're NOT working out.