The reasons I didnt go for a bigger one are that I like to have the flexability to carry stuff on top of this box,I can detach the givi if I want to,to strap big things on,Bags etc fit great betwenn the givi and rider with minimal hassle.
Basically if you look at the 1200 side on ive used "dead space "between the tail peice and the top of the pillion seat this includes the rear grab rail.
When touring just unclip givi and into hotel no hassle,Had BMW top box=shit,don't like panniers filtering=shit
Also I didnt want to spoil the good looks