Why is it that wide angle photos always seem a little disproportionate? Is it because things grow disproportionately small with distance? If so, why is that the case, why isn't it scaled evenly?
I think it might be because with a very wide field of view, given a "slice" of what you can see in that field of view (that is, a plane that is orthogonal to the direction you are looking), a point at the very edge of that slice is much farther away than a point at the center.
With a small field of view, because the angle is much smaller, the difference in distance between a point on the edge of that slice and a point in the center is negligible (sin(angle) approaches zero).
I agree with @KitKasai. Remember here the camera is mapping some spherical view to a plane (the screen/sensor/film). There will be some distorsion depending on the projection the lens does, just like the map of the world.