back pressure can be described as the amount of pressure it takes to keep the unburned exhaust gases inside the cylinder. the more unburned gasses that escape the less power you will make. when a valve opens it is pushing out exhaust gasses and with a smaller diameter piping your keeping the velocity up. with a larger exhaust the the velocity slows and it actually has a harder time expeling the gasses. because denser air is heavier. well colder air is denser. so now becasue of a larger diameter piping the velocity is slowed and it has to move a larger weight to expell the gasses. that is why bigger isnt always better