It wasn't until I took a biology class on the structure of the skeletal system that I realized how complex our bones really are. They aren't just supports for our muscles and organs, they are living tissue just like every other part of the body.
One thing I found interesting was when our professor told us that the weight of the skeleton can vary over time depending on how much force it's subject to. So, if you are a heavy person, you'll end up with heavy bones as well (so that old saying "she's got big bones" has a kind of truth to it!).
He didn't go into it in depth, but it must be this part of the bone that increases, as it says in the article, it makes up most of the weight.