Since I assume you are talking 2 strokes, there is also a simple geometric difference in port area available in your examples. That is one reason why a 500CC single 2 stroke even one that is a direct enlargement of a smaller engine won't rev as high. Think about it this way, the surface area of the cylinder (therefore avail port area) does not increase at the same rate as the displacement. Pi*r2*H=displacement, Pi*d*H=cylinder surface area.
In addition, the size and shape of the ports changes slightly with the various examples. This also has an affect on the power output.
Chris