Its heat ....A HEAVY pellet sits put longer and the falling piston finds a greater cushion of air decelerating its motion and delivers a longer lower pressure pulse with compression generated heat fairly low.A LIGHT pellet will break free sooner and allow piston to fall at a higher rate creating a much shorter higher amplitude pressure spike that sets off a combustion process we know as Dieseling.It is the rate in which the compressing/ed air heats up. Faster the rise in pressure greater is the heat generated.
Why is there ANY smoke with ANY pellet? Something is getting into the combustion chamber and burning.
It is possible you've got a bit of leak by when you cock your gun. Any lubricant on the spring will be thrown onto the cylinder walls when the gun is fired. If your seal is less than perfect instead of scraping all this lube it can pass a tiny amount of it. This tiny amount of lube is then in front of the seal when the gun fires. The resulting hot air ignites it.