Nothing really from the outside looking at it but from the cyl head siting higher up like as well as the carby like wise.
It's just like as if the barrel is longer, although it does not have a barrel that can be un bolted and taken out like, so each one from 450 to 550 to 650 has a longer barrel within the casing, so you can not make a 450 a 550 or 650 etc ect.
Most people can not get their head around that the gearbox is not bigger on the 550 or 650 as they are expecting to see a great big heavy lump like you see with Jap bikes.
I think the 650 is only 1 KG more over the 450, now most people can not fathom that fact.
It's much like a only an extended barrel on a 450 Jap bike engine in theory to save weight.