Variable widths for box plots is a popular convention (see https://en.wikipedia.org/wiki/Box_plot#Variations). Variable width box plots illustrate the size of each group whose data is being plotted by making the width of the box proportional to the size of the group. For example:
http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-369-02/images/pf369-10a.jpg

So, there should be a way to provide pointWidth for each data point of box plot. Also, this feature should work seamlessly with multiple series.

Issue has already been filed to Github: https://github.com/highcharts/highcharts/issues/5441