Most of the spells in the book were ports from the previous edition, adapted so that their level adheres to the new guidelines (which keep the spells at their old levels in many cases). The rules are clear that creating a spell with a smaller target than the individual for that form will not be any easier. To me the limitation of Air's Ghostly Form is deliberate: the spell is intended to cover only the target and its immediate surroundings for a short while rather than swamp the whole area, which wouldn't be any harder, but maybe of a more limited use (and would attract much more attention).
Look at Fog of Confusion, MuAu45 . Size +4 yields "six miles across" a 100x diameter increase from a base 100 yard diameter (6 miles = 10560 yards).