You know, I guess her chest really could be a flotation device if the professor was looking at things from a practicality perspective, and considering he seems to know almost nothing about human thinking, outside of the power of cute, who knows. We honestly don't know much about how Mira's human form works and how fully human she becomes through it, like it can't just be all appearance because she wouldn't be weak in human form and suddenly developing new feelings like love, but we can't be certain she's 100% human either since she can turn back. It's honestly hard to say without more information, especially since even the professor doesn't seem to fully understand stuff like her new emotional developments 🤔