Wow, did Costco ever do a sales pitch on you! LOL
Air is 78% Nitrogen, so, the "particle size" is pretty much going to be the same. So, from that, you can figure your tires are not going to be any "lighter" either (1 lb lighter? How much do you think the air in your tires weighs?)
The only benifit to using Nitrogen over air is that pure nitrogen has no MOISTURE in it, which can affect the longevity of the tire (but who keeps tires for 10 years anyways) and how much the tire pressure increases as the tire goes above the boiling point of water (100 deg C). No water in the nitrogen, so the pressure does not increase quite as much (only makes a 2 or 3 pound diff) (We use nitrogen in race cars because we need extremely stable tire pressures mainly in the 170-240 deg F range where water content makes the biggest diff, almost nobody in a street car ever gets their tires this hot unless they are out of balance) Although, if you use completely dry air (which can be purchased) it is pretty much the same as using nitrogen. Or, just fill up your tires on a very low humidity day for free.
A side note about tire pressures. The size of your contact patch is directly related to tire pressure. Double the pressure, and you cut the size of your contact patch in half. Therefor, if you increase your tire pressure over stock, you are reducing the contact patch, and therefor you are reducing the amount of traction the car has. And when it comes to an emergency panic stop, every square mm of contact patch matters. Plus, any savings from fuel economy you get from running an overinflated tire are greatly offset by increased tire costs because the center portion of the tire will wear out a lot faster...hmm..... get an extra 20 miles to a tank and buy a $800 set of tires twice as often....good savings (doh)
I have learned a lot about tire pressures first from being a class "A" technician, but also from racing, where every 0.5 PSI difference can make a huge difference to tire longevity, cornering ability, braking traction and tire temps.