When the schedule of pipe changes, the added material is added to the inside of the pipe. Why?

Question

Pipe is available in schedule 10, 20, 40 (Standard), 80,

X extra heavy and XX double extra heavy. This is thru 12 or 14 inch. After that, the pipe wall is increased on the OUTSIDE.

Answer 1

The outside diameter must remain the same on pipe under 12 inches because much of it is threaded. Over 12 inches most pipe joints are welded together.

Answer 2

Pipe generally has a fixed outside diameter (OD) and the fittings generally mate to the OD of the pipe (either socket weld or threaded)

Tubing generally has a fixed inside diameter (ID) with fittings that mate to the ID of the tube.

So for a fitting of a given nominal size pipe must "grow" to the inside and tube must "grow" to the outside.

Large pipe sizes are typically butt welded so a constant OD is not required.