mamba
Legend
I assume you got that from this thread, which by now also goes into some detailCan anyone point out the size changes and when they happened?
It literally says in SCAG, page 18:
"Early in 1487, earthquakes and volcanic eruptions abounded for months, as if the whole world was convulsing. Rumors spread of the chasms caused by the Spellplague suddenly vanishing, and stories circulated of known destinations being further away, as if the world had quietly added miles of wilderness between them."
So, no, it didn't occur outside the story in game design - the world, in-universe, did shrink and expand. As ridiculous as you might think it to be, the world, canonically and literally, did resize itself 1487 DR.
For juat the Sword Coast region, leaving off the rest of Faerun:
In the 3E/4E material the Sword Coast region as defined in SCAG is 2.2 million square miles (approximately twice as large as India)
In the 1E/2E/5E material, the Sword Coast region is over 5 million square miles (just shy of Antarctica, the only country on Earth bigger than the Sword Coast region is Russia).
That means the land that disappears and comes back is about the size of Australia. Just for the Sword Coast.