Heres my response to Yana who commented on my previous post - I tried to post it as a comment but had a temporary glitch. So here it us for everyone!
You are absolutely correct with your figures - as we lose weight we need less calories. However, muscle needs more calories per gramme to function than the equivalent weight in fat or other body tissues. This means that if, as you lose weight, you do a lot of exercise and build up muscle mass, your metabolic rate increases a little. Using your hypothetical numbers, if you lose weight from 300 to 200lbs and your metabolic rate drops from 3000 to 2000 - you then do loads of exercise over a period of months and increase muscle mass. Assuming your weight stays at 200lbs, your metabolic rate increases to, say, 2300 calories. If you still continue to consume the same amount of calories as you did before increasing muscle mass, then you will lose weight at a slightly faster rate. This was what I meant when I said I was using my calories more effectively - sorry it wasn't that clear but sometimes my head kind of tips out ideas onto the screen before they've been thought through properly!! Hope that helps!