It's not the area coverage as much as it is the connotations of what is uncovered. For example, though the possibility of Unryuu going without a bra exists with her original outfit, that question has to be introduced intentionally; and, she would still have the same clothing coverage with or without it. The sweater outright removes that ambiguity.
Unryuu-san! What an outfit you're wearing!?This is shameless...Why are you covering your face?That's true...!No problem. Because it doesn't change much from what I normally wear that shows my stomach and my back. Just a little difference, you see.Stomach-killer