Rosalina's chest is big.
I'm actually still drawing that "Solid Rosalina" picture for Tino and I did use these as references, since she is wearing a body suit. And yes, she has a nice bust in my picture. Because she's actually supposed to.
It's just that her normal outfit is essentially a pregnancy gown, which is meant to be really concealing and outright clasps them in (you can actually see the physical creases of her chest being clasped in too, in the HD games especially).
Her other outfits clasp it in too but not quite as much and it sort of pops out sometimes.
The same thing happens with her backside. Body parts don't look the same in every outfit or pose, especially something like breasts which are mostly just fat that can be squished. I mean, consider that these two are the same person:
We can't talk about that much because reasons but I wish people would realise this when talking about VG character designs, specifically with females. There are so many characters that the fandom oversells and undersells in terms of their body proportions.