I just did some digging and made a post on this very topic because I was very curious myself.
Basically it has nothing to do directly with rim size or tire size, but everything to do with the “load rating” of the tire. The OEM tires are an 84 XL (reinforced) load rating tire. If you change to a different size, or sometimes different load rating in the same size, or switch from an XL to an SL (standard load) or LL (light load) tire, then technically you should adjust tire pressures to match.
On the flip side, if you just so happen to change to a completely different size of rim and tire, and it just happens to still be rated 84 XL, proper tire pressure remains the same. That’s usually unlikely in matching diameter tires though, as load rating tends to vary based on aspect ratio and sidewall height.
Mostly it means that moving away from reinforced low profile rubber band tires that require higher pressures to a minus size standard tire means that tire pressures should be lower. You may of course run into annoyances with the TPMS, but Forscan can fix that.
https://www.fiestastforum.com/threads/tire-load-ratings-and-inflation-pressures.29556/