A hard drive has the same volume whether it has something on it or not. I mean, the thing uses magnetic particles or something to put things on the disk. And, it doesn't matter, since it takes up the same space, regardless of whether it has a 1 or 0, or nothing.
Once you write something to disc,and you later erase it, it still remains, only changed. So the disc is basically always "full", in a sense, but with different material on it depending on what is written to it.
It is like having a bottle of air and adding more air, the new air just displaces the old air and the new air is still the same volume as the old air.
Maybe....maybe when new, a hard drive is vacant of stuff, but I don't know. And I don't think the stuff put on it would weigh anything anyway. It is magnetic, and just changes.
Best I can explain it.
You could be right, who knows? But as far as your laptop getting hearvier, the dust on it would probably make more of a difference then what is put on the disk.