Pipes have a consistent ID regardless of wall thickness. In other words, a 1/2" high pressure pipe may need a 2" thick wall, but the ID will still only be 1/2" even tho the OD is 4.5".
Who the heck told you this?
I know squat about tube, but I fit pipe at least one day a week, for six years.
Pipe has a nominal size, and a "schedule" size, or wall thickness.
The OD is constant (a little bigger than the nominal), so the pipe will always fit in the same fittings. Then the higher the schedule, the thicker the wall, and the smaller the ID.
Now I suppose that with a small enough wall (sch10 maybe?) the ID might get as big as the nominal size. We use mostly sch40 and 80, so I can't say for sure. But I can say that the "constant ID" just ain't so.
And one other thing: some of our presses run to thousands, even tens of thousands of pounds of pressure, all in sch80 (about 1/4-5/16" wall). I can't imagine anything needing 2" walls on 1/2" line.
Sounds made up to me, by someone who doesn't know what they're talking about...
Robert