And most of the people who play as Rosalina happen to be guys. Most girls I see stick with a baby character, or Peach and Pink Gold Peach (the latter of which is actually heavier than Rosalina). I've also noticed that a LOT of these guys are Japanese. Which makes things even more understandable.
Just note that I say this as someone who is, in fact, very sexually attracted to Rosalina (though I do genuinely love the character a great deal and I play as her because of that, and because of the fact that she happens to be the best character in pretty much every game she shows up in).
...I'd say it's a fairly big difference. We're talking going from flat chested Peach in the actual Mario games, to at least a C Cup in Smash. Her TV Tropes page for Smash even puts her under the "Hotter and Sexier" trope.
Roy and DK have near identical stats to Rosie and Waluigi, the difference is basically negligible. They are the same class, just ever so slightly different. I think Roy and DK have a tiny bit better handling than Rosie and Waluigi, while Rosie and Waluigi are a tiny bit heavier. Or it could be the other way around. The difference in either situation is negligible.
I didn't get to play MKWii online but I am aware of how unbalanced certain characters were. However, MK8 is much better in that regard. Rosalina's weight class (Cruiser Weights) is considered the best by many, but there is room for preference this time around, and due to how items are handled, online races end up being really luck based at times anyway, meaning a good combo means little. For example, I much prefer my kart combo for Rosalina than the Sports Bike one people always use:
One thing to note - Acceleration is tiered in MK8. "2.50" acceleration is the exact same as "2.00".
Also Waluigi is a popular character outside of MK in general, there was a lot of outrage when he was confirmed an assist trophy for Smash.
Though to be honest, I don't see why you would need to defend the notion that she is not sexually appealing. There is nothing wrong with it, as she is an adult, human character, and a very beautiful one at that.