Reading the 1e
Kara Tur commentary it appears that the geographic change was between the 1e FR Kara Tur and the
2e Horde boxed set:
Exploring the Realms. Kara-Tur is placed to the far east of the Forgotten Realms. The map included here doesn't yet connect to the maps of the west; that would await the Empire event. When that connection appeared,
The Horde Barbarian Campaign Setting (1990) would reveal another problem: Kara-Tur is just
too big when compared to the rest of the Realms. To accommodate this,
The Horde would reduce the scale of Kara-Tur's maps by one-third to better link up the west and the east.
And from the Horde commentary:
Kara-Tur. Kara-Tur had one large problem: it was too big. This required resolution now that
The Horde was linking Kara-Tar to the western Realms. Cook explains, saying "Because of the immense distances involved in Kara-Tur, the overall size of Kara-Tur has been reduced by one-third to improve play. The 90-mile scale maps provided in the Kara-Tur set should be revised to 60 miles to the inch. The 30-mile scale maps are reduced to 20 miles to the inch. On the plastic grid, one small hex equals 12 miles on the larger maps and 4 miles on the smaller maps."
It looks like this is just metagame rescaling the maps and not a physical in-world change of the size of the world through a contraction of the planet during the Time of Troubles or whatever.