Maybe Benz designers think (as I do) that every lane change requires an eye check, not just a mirror check. The passenger's side mirror is the smaller one and should be relied upon less than an actual viewing. Sometimes I don't even use it for a lane change because I've done headchecks. Of course, that's not really a good explanation for why the driver's side mirror should be larger.
I'm sure you've noticed that in W124s the driver's side mirror adjustment is mechanical while the passenger's side is electrical. Ever wonder about that?
2003 Dodge Durango SLT
2002 BMW E39 530i Sport
1988 Mercedes 300TE
2003 Chevrolet Suburban