Why do aircraft of the same model get progressively larger engines as they mature?
The Cessna 172 first flew in 1955.
The Continental IO-360 was FAA certified in 1962.
The math is pretty simple.
Yes, we (Americans, at least) are getting fatter, but that still doesn't allow for use of an engine that isn't yet certified.
It's the same for GA as well as commercial aviation. Improvements in fuel efficiency and power tend to lead the manufacturers to want to certify existing airframes for new power plants because that's less expensive (testing, certification, pilot training, etc.) than building a whole new aircraft around a new engine.