That drop would be so slight that you wont be able to measure it with any consumer grade devices. Yes, the molecules are slightly smaller, but even if what you stated about O molecules escaping easier is true, then wouldnt it imply that you will end up with 100% Nitrogen in your tires automatically by simply keep refilling the tires with regular air (because the rate of O escaping would be far greater than that of N)?
Nitrogen filled tires is a scam. There are legitimate reasons, why you want N filled tires in an airplane or in a race car. But none of them apply to everyday driving.