I think if you had to scale up Drogon to a similar size to Smaug, including proportionally thicker bones, scales etc, it would potentially be an interesting fight. As it stands, there is just way too much of a size gap there.
Of course, there are dragons who can punch above their physical weight class, like Alduin on that list, for example, but that's because his power stems from his magical abilities, not his physical ones.
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