At the end of the day, the facts are the facts; a 25mm tire experiences less rolling resistance than a 23mm tire at the same air pressure. Without cutting too deep into the science of the matter, it all comes down to the contact patch. To paraphrase an insightful discussion on the matter between Jared Gruber and Wolf Vormwalde for Peloton, at equal pressure, a 25mm tire has a “wider but shorter” contact patch, while a 23mm tire has a “slimmer but longer” contact patch. The increase in the required deflection of the latter example equates to spending more energy “into deforming the material.” This is pretty well demonstrated by this chart from Continental: