Based on my experience, the helmets do break in and loosen up a little over time. So, I would recommend buying the smaller helmet and seeing if it loosens up. Also, some helmets allow you to buy smaller or larger check pads and head liners (I know Shoei and Arai does) to customize the fit.
In my case, I had an Arai VX Pro III that seemed to fit perfectly and then once I started riding it started to loosen up. I bought a thicker head liner and cheek pads but it was still a little too big, although much better. Ultimately I ended up getting a new Shoei helmet in the next smaller size and it feels tight at the moment but I am confident it will loosen up. If it doesn't I can buy smaller padding.
Good luck.