What does the "mass" of a box plot mean? 
By “mass” I mean the size of the two subretangles that make up the box in a standard box plot. I thought the mass quantified the observations below and above the median but if it did so every box plot would always be symmetric, because 50% of the observations would fall below and above the median. If so, then the mass would be the same below and above the median line.

The box represents the range from the first to the third quartile, so it only covers about 50% of the values. As such, the median bar does not need to reside in the middle.
