Tire pressure drop has nothing to do with the tires, unless you are using tubulars, or even tubeless tires. This is not your case. The pressure drop is due to the inner tubes you are using. The lighter the tube, the faster it loses air. If you replaced your tubes with thinner lighter ones when you replaced your tires, that is the reason your tires are losing pressure faster. Tires should be checked not by the amount of mileage you do, but according to the amount of time that has passed since you topped them up, the lighter the tube, the more often you need to check tire pressure.