Actually, Rosalina's chest looks a fair bit smaller than it actually is in most games.
The gown she's wearing clasps her breasts in pretty tightly. It's not very noticeable in most games - but in 3D World, they look really "squished up" (it's also noticeable in Smash but not as much due to the shiny texture of the gown in Smash). There is a very defined cleavage, but it looks like her breasts are being pulled up pretty tightly.
I'd reckon if she let them totally loose, they would be between a C Cup:
She would not be spot on either end, but probably closer to a D Cup.
It should also be noted that this is assuming she is her size from 3D World (a pretty standard female size). When her regular 7'8" size is being used, they are scaled to be even bigger. Technically, they are actually really huge, even bigger than Samus'. However, in relation to the rest of her body, they are a high-moderate size.
....I actually wrote-up a piece about a pet peeve of mine - where people like to draw Peach with a high-moderate size (if not bigger), but Rosalina with a flat chest. It's actually the other way around in the Mario games (but Peach has a size more similar to Rosalina's standard in Smash Bros, maybe even slightly bigger if you scale the two ladies to be the same height as each other). I don't dislike this because I want Rosalina to be hotter or something (she automatically is to me anyway, and in NSFW art, proportions of all characters tend to be all over the place anyway because people care more about having wildly diverse body shapes and satisfying kinks/fetishes, than they do with representing characters with their canon proportions), it's because that's not how things actually are (meaning it bothers me in SFW art as opposed to NSFW, where I don't mind). Peach has more petite features and it's done that way for a reason - because she's sweet, juvenile and innocent in her personality. Rosalina, on the other hand, isn't a completely innocent figure considering she's protector of the universe and has that tragic past where part of it is her fault, and in general, has much more adult qualities, like being reserved, a maternal figure, etc. Therefore, her various "assets" are more adult in proportion, while Peach barely looks legal outside of Smash.
/characterdesigncorrections