The weigh in might just be telling us something here. It is always a bad sign when somebody weighs in with their shirt on. If Chisora is taking this more seriously than Fury, he might well reverse the result of the previous encounter. Of course Fury is sly enough to do something like this to throw Chisora off his game.
Exactly that, a pound of muscle weighs the same as a pound of fat. It's the same as that old thing about what falls faster a ton of coal or a ton of feathers? Muscle has more density than fat, so 1 pound of fat weighs the same as 1 pound of muscle but the muscle being more dense takes up less volume.