Hinterlands

Isn't there a set distance between the Spire and the edge of the Hinterlands? How can infinite divine realms be fit into that area? Or do some deities in the Outlands make their realms in the Hinterlands?

As noted above, it's a property of the Outer Planes that gross distances don't really matter. Let's assume for the sake of argument that the distance between the Spire and the Hinterlands is 5 miles in every direction. Mathematically speaking, we should expect the habitable area of the Outlands to be, at maximum, 78.54 square miles. But on the books we have... what's this? 150 square miles of documented locations! And what's that place on the horizon that doesn't look like anything on record...?

The Outer Planes are realms of belief; they don't conform to any need for fixed areas to exist. While certain spatial relationships are more stable than others, it's far more likely that on the Outer Planes the distance between Strangeland and Otherplace is literally "two days' travel" than that it is any fixed distance in miles or kilometres or linear tessipates or foot-cubits. While there is a fixed distance between the gate-towns and the Spire or the gate-towns and the Hinterlands, all that area in between is whatever it needs to be for whoever is traveling.