Hmmmm. Great comparison, and even better pix!
I think you could fit the M5 inside the 7 though. Every dimension on the 760 looks bigger. Not by much, but it makes a difference.
I share your puzzlement about the perpetual growth of pretty much every model from every manufacturer. What will BMW do when the 1 series is as big as the current 3's and they need a model to fit the 'nimble' category? 0.5xx?
I'd buy the M version of that! M.5...