>>26571307The map of Sinnoh you posted looks pretty big, but compare it to the actual gameplay map of Sinnoh here. Note the large swathes of unused space between the actual map parts. Note how much of the actual map is walls of trees or rocks.
Based on the first island of Alola and what we've seen of its gameplay, it looks like the region will have almost none of that wasted land space compared to past regions. Everything depicted on the map seems explorable, and so I would expect Alola has about as much content as any other region, but in a more compact space.