I think you have answered your own question...
Individual preference, Horst likes super small short boards, others (most I think) prefer something in between, and of course, the highest jumpers are the racers, can jump much higher because of more speed, big foil kites, and good technique, and almost perfect boards.
So they use their normal raceboards, eventhough they are not small - but you can jump higher on what you are used to.
And raceboards are superlight because they are thick and with carbon, and stiff, so performing max, giving the highest jumps and easy to rotate in the air as light.
Even smaller boards could rotate easier, so some prefer this, but would not make sense for racers not to use their almost perfect boards IMO.