Even if for some strange reason the dealer installed a smaller filter your oil flow rate would not decrease. There is a bypass valve in the filter adapter on the engine block that opens and allows oil to bypass the oil filter if too much pressure builds up. This is to prevent engine damage in the event that an oil filter is plugged or partially plugged. A filter that is too small would behave similar to a partially plugged filter; some oil would bypass the filter due to the higher back pressure caused by the filter.