All I know is that before I started riding I spent some time as a passenger riding with my brothers in CA. One time I did a day ride, borrowed one brothers M (Shoei RF series) and my head hurt like made, the top of it on the inside was pinching my skull. Next ride I borrowed other brother's L in the same model, and it fit quite well. When I first started riding I looked at helmets, especially Shoei since I knew they fit my head shape well. (My brothers and I all have larger rounder heads.) I tried a M again, walked around the store a while, within minutes I had a splitting headache. I'm not riding like that, if the Large is too lose I haven't noticed. It does move a bit more than some say is optimal, and my cheeks aren't pinched by the pads. I've been lucky to not test a helmet yet other than low speed stuff, so I'm not sure about how they will do.
But I am NOT going to let somebody tell me "no that's too large wear the smaller one that will give you a headache, it will break in eventually".
Yes it is a lot like oil, I try to pick something, stick to it, change it regularly and not be swayed by every new study to question what I'm doing. I can be neurotic enough without that stuff
(Its like the debate about eggs, are they good or bad for you, or many other foods.)
PS Riding season did kick off a few weeks ago, but since then last weekend and this coming one are washed out, just too cold, or worse SNOW!