Perspective Compression
The apparent flattening of distance between near and far elements when photographing from far away with a long lens.
In depth
The effect belongs to camera distance rather than to focal length: standing further back reduces the ratio between the near and far subject distances, so their relative sizes converge. The long lens is simply what makes the distant framing usable. This is why a telephoto stacks a row of buildings or makes the moon loom over a skyline.
Examples in practice
A 400 mm shot from a kilometre away makes a line of houses appear pressed together, each barely smaller than the last.