I don't think that that's quite right. SCOTUS opinions do indeed use a narrow text size (4 1/8" by 7 1/8" [0]), but that's because they're designed to be printed on unusually narrow paper (6 1/8" by 9 1/4" [0]), not to leave room for taking notes. This paper size actually only leaves 1" margins on either side, which leaves less room for taking notes than most other documents.
That they can lop those margins off in the final printing that goes in a bound book is just saving a bunch of paper.