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The general assumption that two-stroke bikes rev higher than four-strokes is not supported by this group of bikes. With a bore and stroke of 56 x 50.6 the NS has over-square dimensions. The lowest revving bike in the group is the NS250, the only two-stroke in this row of Honda motorcycles, the piston is traveling about 3,486 feet per minute at redline, which is slower than the pistons of the other 250's. Considerably slower than the CBR 250's tiny pistons. 
The largest displacement of these four bikes, but with four cylinders, each cylinder on the CBR600 displaces only marginally more than the cylinders of the two twins, the NS and the VTR, even though they are 250's. The size of a piston is one of the factors limiting the speed at which it can move, hence the RPM that can be attained. 
A liquid cooled v-twin Honda motor, but unlike my NS250, Mel's VTR is a 4-stroke. The VTR revs somewhat higher than the NS, although the cylinders are the same displacement, the bore and stroke dimensions (60 x 44.2 for the VTR250) are different, and so the pistons are different sizes. Both the NS and the VTR were originally built to be sport oriented street bikes, and have since been converted into vintage race bikes.
This little Honda revs to a spine tingling 18,000 RPM. With four tiny (62.5 cc - about 4 tablespoons!) cylinders, very small pistons, and valves smaller than some body jewellery I've seen, this bike can rev high. With a bore and stroke of 48.5 x 33.8 this motors dimensions are over-square like the other 250's, but the potential for high rpm operation for this motor is far greater than any of the other motors whose tachometers are pictured above.