I love how Mami's tits get keep getting bigger and bigger in fanart. They weren't that big, lol.
Not quite surprising though, any character labled as having large bust in comparison to the rest of the cast(AKA becoming a "token busty girl" of the franchise), regardless of how actually large they are portrayed to have in their respective media, will always have exaggerated sizes when it comes to fanart interpretations. Mami's fanarts actually depicted her as having breasts larger than her head ever since back in 2011.
Not to mention that this particular artist just draw any girl not labeled as "flat" as having huge breasts anyway, observe his images of Sayaka for example.
You know, that's pretty common. As soon as a character is labeled as having the largest bust of a cast, even if it's modest at best, the fan art tends to run with it until it gets ridiculous. Not that fan art has ever truly needed a reason to get ridiculous.