So is _Sapeornis_ the largest known FLYING Mesozoic bird?
The largest known EARLY Mesozoic bird?
The Mesozoic bird with the best press agent? Or what?
It was larger than any known bird before it, and larger than all its known
contemporaries among birds. In other words, it was the first to reach a
certain size.
(All of this can depend on the definition of "bird", it goes without
saying.)