I some EU countries 125cc are limited to 80kph for under 18 years. In this case a strong 50cc may be faster downhill squeezing out (and risking!) it's engine.
But usually 125cc are unrestricted too and can utilize their <=11kW/15hp. Top speed then about 110kph.
I own a small 125cc with "only" 7.5kW/10.2hp. It looks small and from the front and side like a 50cc, some people like to race me. But it outperfoms every 50cc scoot I have ever met. Once they see my tail with the big number plate, they give up. 150% _more_ displacement (unrestricted anyway) is nothing to take lightly, even if it's "only" 4-stroke.
In switzerland the 50cc may legally run unrestricted if the driver has license class A1. The strongest 50cc has 4.2kW and a top speed of 78kph. Compare this to 11kW and 110kph.
We have an 125cc class with >11kW too, for driving license class A. 4-stroke may reach 20hp (Peugeot Satelis K20), 2-stroke 23hp (Aprilia RS 125).
Who thinks, an unrestricted 50cc can compete with an unrestricted (but stock!) 125cc of 20hp and more? Who's unrestricted 50cc runs true 120-130kph or even more? "Speedometer tuning" doesn't count!