It has always been my understanding that the reason is the angle of view.
From the driver's seat, you are close to the left mirror and need a wide angle of view, this the mirror is wider/longer.
The right side does not need to been wide/long because your view angle is narrower.
This also plays well into why fast cars in Europe, especially Germany, will only have left outside mirrors. According to the factory, each outside mirror will lower top speed 3-5mph.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)