Rosalina being lightweight wouldn't even make a whole lot of sense because Luma is so easy to K.O as it is.



The whole point of her playstyle is that you're worrying about whether or not Luma can survive. Get one good hit that sends him off a bottomless pit, and he's gone, as he falls straight down, and will be helpless until he lands on solid ground. When Luma is gone, Rosalina loses her tag team gimmick for 8 seconds, as well as most of her K.O potential, near invulnerability to grabs, her masterful control over the battles pace, her ability to set up really elaborate combos with Luma, etc. That's the risky side of her playstyle, unlike Mewtwo who was light and huge himself. Rosalina is still able to survive perfectly fine, it's Luma that you have to worry about, since losing Luma will put everyone but the best Rosalina players into a position where they feel they have to stall, thus meaning that her opponent now controls the pace of the battle.



This is something that has been well known for a long time, and many people in the Rosalina boards have been asked this question time and time again. And we reply with the same answer every single time - "she's not light, just floaty, like Samus".



I think her being "light" is being mixed up for her floatiness, because floaty characters are easier to K.O horizontally, regardless of weight, since that floaty effect means it's harder to cancel momentum in the air and as such, the just get carried horizontally (unless the character has momentum negating moves, which funnily enough, Rosalina does have in her Down B - hence, she's an excellent survivor). However, it's harder to K.O them vertically. Samus is also like this, she's easy enough to K.O to the side, but not towards the top of the stage, and even spikes are less effective on her.