Because the actual dimensions of the map havent changed, this has caused the Eastern Kingdoms to shrink in order to create space for the new easternmost landmass, as it would otherwise fall off the map or force everything else to move west.
As seen below, the old map made the Eastern Kingdoms look quite a bit larger relative to other continents. Previously, the Isle of Quel'Danas was in line with the southernmost parts of Howling Fjord, while it now barely intersects The Azure Span. Likewise, Stranglethorn Vale no longer falls in line with Kun'Lai Summit on the main continent of Pandaria, but is now closer to the Isle of Giants. Reducing the Eastern Kingdoms this way has also created a wider gap between Tol Barad and Kul'Tiras, and helps show just how large the new zone coming in Patch 12.1 is compared to the rest of Azeroth.
