I think the numbers will be different depending on which basis you use. But I think that whichever way you run them, the result will be that 2023 is more cramped on average than was, say, 2015 or 2000. For sure the average passenger size is going only one way.
I skipped over the situation with widebody jets. Mostly because I don’t have any source for hard facts and not enough personal experience to extrapolate from. For 777 / 787 / A330 / A350 sized jets, they’re wide enough that the carriers can either use e.g. 10-abreast with narrow seats or 9-abreast with wider seats in coach. Plus farther forward they can do 8- or even 7-abreast with biz-class seats or 6-abreast with first-class lie-flat modules.
Which carriers do their coach seating how on which routes I can’t hazard a guess. Nor how that may have changed over time. For sure the growth story of the last 20 years in worldwide aviation has been in China, India, and south or southeast Asia in general. Those are much smaller people on average. So perhaps the carriers are stuffing in a lot of seats but the people still fit. Any worldwide average would start to lose focus and hence decision-making utility pretty quickly.
I really think that for Joe and Jane Average US domestic air traveler, most of the shrinkage they have suffered is the replacement of 2 727s per day with 6 RJs per day from their podunk hometown to the hub. And in many additional cases it’s from substituting e.g. Allegiant or Spirit where they had flown e.g. American or Delta (or their many predecessors) before.