So what exactly is the difference between these and the I/O? From what I can see the only difference is the shape and the size. Which will fit me better, the I/O or the I/OS? I'm wearing a Variant Brim in a large size. Yes, I realize this is hard to assess, but exactly how small are the I/OS?

I wear a L helmet and I just went to the store and tried on the I/O. The curvature was huge and I could see a huge amount of daylight through the nose bridge. I think the difference isn't necessarily that the I/OS is smaller, but I think the curvature is less dramatic. Therefore, it fits women who generally don't have as big of schnozzes and flatter faces (like asians). I know this because I own A Frames and they're made for "smaller faces" and they fit me perfectly even though I wear a size Large helmet.

I to rock a size large helmet and I have the I/O, fits my face and head better. Order one try it out and then use the bomber return program through backcountry.com if they don't fit. Problem solved

I wear a size Large helmet (the Smith Holt) and i wear the I/OS with it. I tried on the I/O and it didn't fit my face at all. The bridge of my nose was apparently way too small for it, there was a huge gap there. Which surprised me because even when goggles dont fit well gapping at the bridge of my nose was never the issue. I can wear Oakley Stockholms, A-Frames, or Smith Phenoms no problem. So i guess i have a medium/small face size for goggles. But yeah, like the person who commented previously mentioned, knowing what goggles you've worn in the past would help determine which one would work better for you.

I think you answered your own question - the I/O S are smaller. They're not a whole lot smaller. What other goggles fit your face well might help us figure out which would be better then you. But if you have a large noggin (helmet), I'd think the regular I/O would be a better fit for you.