Some bikes are corked up it's like my car. I take the stock air cleaner assembly off and put a cold air intake on and the car seems to run better. Although there's another problem the exhaust is now restricting the engine to pump efficiently, so what do we do, you guessed it, put some headers and bigger exhaust system on. Now the engine is really pumping air(think of an engine as one big glorified air pump), then there is yet another problem, the damn thing is running lean as hell. Well, you have to jet or tune the engine to run richer.
The two stroke is the same, if anyone remembers the RZ350, they will remember those restricted pipes with the catalytic converter built into them. They sucked up a ton of power, answer to the that, see above statement.J/K People would make there own pipes or get the uncorked Euro pipes.
In the end the engine, no matter what stroke is a air pump, it pumps air like a water pump. There is some math to see how much air your engine pumps in one stroke, but I'm not going there.
The experts say a modern 125 two stroke pumps around 173cc of air burned each revolution. A lot of air in a small engine.