Roadmarks, while you are correct that the internal volume changes, it doesnt change by a noticeable amount, because for every piston going up, one is coming down. The minute variances are due to piston slap, etc.
The main culprit is blow-by, and on older engines it is more than new engines due to ring and valve seal wear. The crank case vent is meant to expel the gases. The metal mesh inside gives the oil vapor a place to condense and turn back into oil dropplets. For todays engines, the PVC (Positive Crankcase Ventilation) valve is used to draw engine vapors into the intake to run them through the combustion process.
Burning the tar-like oil out of the canisters was common. So was soaking in gasoline or other solvents.
For diesels, a passive system is still used today, now with a small drain tube back to the oil pan to catch the oil.
Turbocharged engines expel more blow-by as BMEP's are higher.
Hope this helps.
