Giro Advantage 2 to small?

My initial reaction with the Giro Advantage 2 was that they ran small. I take a medium in a Giro Ionos and a Medium in a Bell road helmet also. I'm wearing the M/L size in the Spiuk Crono aero helmet, which is a "one size fits most" helmet.

When I tried a large it was absurdly big. Firstly, I looked like the Great Kazoo and secondly the ears weren't much looser than the medium- it was just the darn helmet was too large overall.

I'm told the ears on the Advantage 2 should be snug to maximize aero performance and prevent a lot of wind from getting between the helmet and your head. That makes putting it on in transition something your want to practice before race day. Also, your sunglsses go on aftr the aero helmet- another thing you want to run yourself through a few times before race day.

Ultimately I found I did wear a medium. The large was simply too big and the ears weren't that much looser. Apparently, part and parcel to an aero helmet improving aerodynamics is preventing a lot of air from getting between the helmet and the head. Giro snugged up the fit of the ears on the Advantage 2 to facilitate that.