Simply the best option for Rosalina to Drive
I thought of the REAL proportions of that bike, considering that characters like Bowser ride on it.
(I still insist Rosalina became a heavy weight character due to the drastical change of what it is the earth's gravity and teh gravity on Star Road. I say she had to increase her "gravity", so she could stand on her feet when on earth, since she is always floating. That made her heavier than the other princesses, in spite of the fact that they all have similar heights.) -
...and there goes my crazy hypothesis. That's why she looks so small compared to that bike