My God, there is still somebody out there that hasn't understood the whole 1 cylinder, 2 cylinder, 3 cylinder, 4 cylinder, 5 cylinder, 4-stroke vs 2-stroke limitations?
A twin isn't going to make as much horsepower as a four, and it all has to do with the physics of air flow, more combustions, etc. By your question why don't you ask "shouldn't a DR650, KLR650, or XR650 blow away the measly 600?" How come 600s make more power than huge 1400 cc twins? Pure capacity does not translate into hp. Hence WSBK rules that gave weight breaks and capacity breaks to not only twins but triples too (I guess trying to get Triumph in). Hence the current MotoGP rules that give significant weight breaks to twins/triples, and penalizes 6 cylinders compared to fours/fives. Why do you think Honda went with the five? On the same side it takes a 1000cc four stroke to make as much power as a 500cc two-stroke. Why do you think motocross series allow double capacity four strokes in 125 and 250cc series?