If liter bikes and everything in between all weigh about the same?

 

While weight may have something to do with it, I think the term middleweight more commonly refers to the 600-800 cc sportbike while 900 and up are referred to as Litre bikes. The lightweights would be the 250-500 range. You can certainly see by comparing a Ninja250 to an CBR600RR to a Ducati1098 that the actual mass is not all that different, but the power output and racing class are. That is my take anyway.