Stupidly so, in fact. You know the kind that hentai doujin writers give female MCs?
Yeah, that big.
Normally I wouldn't call hers' ridiculously big. There are real life women with that natural bust size and is not considered to be handicapped.
It's just that her size in proportions to her shoulders and torso are so off that they looked more like she stuffed balloons into her shirt than they're actual breasts.
In fact, given that she had yet to show her cleavage in official arts, it's actually my headcanon that those are balloons.
Yeah I think the most unrealistic thing about Ryouka is that the rest of her body doesn't match. There are teenagers with boobs as big as their head who aren't fat, but they've usually got epic hips to match and their torso isn't so thin.