My observation is that on the larger planes, the servos are located closer to the ttask at hand to keep the connection tight and free of as much slop as you can. With that said, generally there no add'l "stresses" put on the throttle arm from wind, etc so I'm not sure why so many mount the throttle servo so close to the motor. Besides easier hookup by not having to go around foam encased tank for example, I'm not sure. Maybe it makes for a neater installations since the large planes can have add'l equipment like redundant batteries and other stuff not generally associated with smaller planes and it leaves more room.