Why do all bikes use about same size carb?

[SoFlo]

I've been wondering why basically all modern 125's, 250's, and the 500's all use basically 38's or 39's. A 500 has four times the displacement, why the same size carb?
And off road bikes seem to use slightly smaller carbs. And two stroke ATV's use even smaller carbs. Anyone know why this is technically speaking?

 

[250racing]

I've been wondering why basically all modern 125's, 250's, and the 500's all use basically 38's or 39's. A 500 has four times the displacement, why the same size carb?
And off road bikes seem to use slightly smaller carbs. And two stroke ATV's use even smaller carbs. Anyone know why this is technically speaking?

in theory smaller carbs work better at low rpms because the smaller bore needs less suction to fill up the cylinder with fuel and air. (thats why the offroad 250 and 300 ktms run 36mm carbs)

a bigger carb is better suited for top end thats in part why 125´s are pipey but have great topend power usually run 38 mm carbs. they need rpms so that the engine sucks enough fuel and air mixture to make it run good.

2 stroke atvs run small carbs because their engines usally dont have power valves and the are lousy off the bottom. so they run smaller carbs to make more rideable.