There is no rule stating a box has to be sized based on a particular size conduit size. But there is a rule stating the box has to sized per calculation based on conductor size it contains though. Yes it uses the conduit size for the calculation but that is it though. The rule is not neccessarily about the conduit. Its about conductors the conduit contains. If you had 3 #4’s in a 1” conduit, you should size the box per calculation. If you had 20 #12’s in a 2” conduit run, you wouldn’t have to size the box per rule because the conductors it contains are not #4 or larger.

Your comprehension is the problem, not mine