Actually Rosalina is heavy.
Mario Kart may use a size based system, but it's not like Rosalina is very skinny. She's near 8 feet tall, has fairly thick thighs, voluptuous hips, backside, and a big bust (her gown hides it but you can see it in certain positions in her biker outfit), and she has a big head. It's not hard to believe she could be heavy. Cruiserweights in Mario Kart are still considered heavies, and she still shares the class with Donkey Kong.
If you need another game that proves Rosalina is heavy, look no further than Mario Golf: World Tour. For her Double Bogey animation, three Lumas try to keep her standing after she faints, but they can't hold her and she drops like a rock. She hits the ground with enough force to cause a small tremor and sends the Lumas around her flying.
The only reason Rosalina is light in Smash (even lighter than Peach) is because she uses her gravitational magic to make herself lighter for fairness, as Sakurai stated. You can see this magic at work in Rosalina's single bogey animation too, in World Tour:
The moment the single Luma catches her, she wakes up and turns on her powers so she doesn't fall on top of him.