You are right that the size difference isn’t that big. But for example the 11” got 2.1 mm taller (in portrait orientation), which means they would have needed to position the Smart Connector 2.1 mm higher in order to still align properly relative to the camera bump. In addition, if you look at the
Accessory Design Guidelines, the magnets are arranged in a fine alternating grid of 4.5 mm. In the new iPad Pros the magnets would have to be repositioned very carefully, and slightly less symmetrically relative to the frame, to still match the old layout relative to the camera bump and Smart Connector. Not impossible, but also not trivial.