I bought my usual Castelli size and I must say it feels strange not having my midsection squeezed while wearing it, my gut isn't that big but not flat by any means. The quality is as expected and the fabric is comfortable, sizing down (can't believe I'm suggesting that for something Castelli makes) will probably get you closer to a race fit if that's what you're looking for.
Another great product from Castelli. You might pay a little more but worth the extra spend. Very comfortable and I ordered one size up to have a little more room. I really like the color and definitely my "go to" short sleeve shirt on any ride.