A 1000 CC vtwin has twice the displacement per cylinder that a I4 does. Displacement = Bore*Stroke. So in a V twin each cylinder will have a larger bore and stroke. A longer stroke will give you greater torque, its like using a longer lever to move an object. The longer the stroke however the farther your piston has to travel, which means a higher piston speed per revolution. This is what limits a V twin to lower RPM's.
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