The amount of trapped pressure released is still a function of the exit area (the breather), but the can has 2 advantages:
1. more oil will settle out (the air inside the can is "quieter"), thus less oil goes out the breather
2. the extra volume inside the can slows down the "huffing" effect of the pistons going up & down and averages the pressure (the crankcase volume varies constantly due to rod geometry - the pumping never balances out)
