Different timings, different gearbox, different exhaust, different carb....
A 50cc bike can go up to 140MPH (It has been done) depending on how it is build.
The minicross is build for speed, not for reliability. A normal moped is build for reliability, not for speed. This is a huge difference.
Depending on how the 50cc and 80cc is timed, the other components used and the ratio of the transmission, a 50cc can be faster then a 80cc.
Why you're bike is quicker now, is because you actively increased the gear ratio. You fitted a larger wheel and you increased the amount of turns it makes per rev (Longer gear ratio) so you've increased the top speed. You can only do this so far as you have enough power in the bike to pull that ratio. A 35 tooth sprocket might be pushing it, but I don't know how much power you have left.
One thing is for sure: If you install a longer ratio, you will reduce acceleration.