I don't recall there being more than one or two routes between the cities after Shalour in Kalos, either. They still made it work. They can make this work, too.No, you have a point. But still. Especially Melemele and Akala look extremly small. Melemele takes a long time, since its the starter/learning island with many cutscenes, while Akala feels so small because all three trials follow so quick after one another. Take away the cutscenes and Melemele and Akala can be done extremely quick, since the spacing of routes on Ula'Ula and Poni have been done better imo. Three trials on Akala also feel extremely cramped, if you take the size difference between Akala and Ula'Ula into comparison it would have made more sense for Ula'Ula to have a third trial.
In the anime, you feel they are stretching out the School Setting as long as possible, so they don't hit the next island to early.
Also, I was under the assumption the school stuff on Melemele was dragged out because they were waiting for USM to be announced. If they continued on, they would have fallen into the same pitfall they fell into in BW. USM would have unexpectedly been dropped on them, and they'd have to change things in order to market the new games.