Tire casing do stretch over time. Most of the growth occurs with the first 24 hours of initial inflation, and of course the higher the inflation pressure, the more rapid the stretch. Tire construction also plays a role; a 2-ply DH tire will not grow the same amount as a tire with a single-ply 120 tpi casing. When they are new, you can install them and leave them inflated to maximum PSI labeled on the sidewall of the tire for a few days to stretch them to max size, then air down to a reasonable pressure before riding.