All to do with airflow
Im not a mechanic but im just interested (Im a Chef by trade)the difference is the rate of airflow dynamics and sequence of revolutions I think. with the v and flat motors the makers are able to get much more airflow and direct ramming(Turbo & Supercharging)without splitting the airflow which creates horsepower, but you sacrifice smoothness(Inline) the motor is actually a large air pump the more air that is fed through the motor the more power is created.
Thats why the japs run small motors in WRX"s and EVO"s but produce high HP And KW The Subaru is only 2litre but has phenominal power for small car (It has boxster layout like the Porsche also) but is turbo charged .
The newer benzes are comming out with V6 motors with supercharging and they are reasonably economical and have great torque so im told.
Maybe there is some tech out there able to verify these pionts im not totaly certain if this is correct (Im only a Chef not a mechanic as i said, just interested)
W123 280Te 1982 Modified & still going
W210 190E 1990 (Wifes Car)
W116 350 se sold
W116 280 se sold