I'm quite sure I read a quite recent study (few years ago) which said that 1lb of fat ends up burning almost as many calories as 1lb of muscle (probably TDEE though, not BMR). I'm too lazy to find the source though, but if you/someone is interested, you'll probably be able to find it by googling. Edit: Googled anyways, the original article seems to be gone, but google "Dr Cedric Bryant muscle fat calorie burn" and you're going to find a lot of articles quoting it.