Dude, I feel your pain. I'm a pretty thick-type guy, (I prefer husky ;D) 5'10" 200+ pounds depending on how satisfying my morning BM'er was. And I have a large, thick like a brick, square skull. I need an XL in almost any brand helmet you care to name.
I had a few awesome, seriously awesome, vintage lids, that were nice and slim and didn't make me feel like Darth Helmet.
I also have a nice HJC IS-16, with the retractable inner sun visor. And I swear to whatever you wanna swear by when you almost have a get-off at speed, you will never EVER care about how much bigger your nice modern helmet, with the thick ass padding and assorted other crap, is after you almost eat shit at anything approaching pucker-factor speed. This is one of those lessons you want to take our word for. Learning it for yourself sucks. Watch some other dude learn it for you, then visit him in the hospital just to make sure you get it.
Get something comfortable, shop around, drive clerks crazy trying on a bunch of shit. Ignore the Harley twat, she's probably gonna burn her calf off on some jerk's tailpipe at least three times before she gets that skin grafts aren't the new sexy. Find yourself a nice hot chick all in skintight leather, with a matching giant dome, you'll be happier. And better looking.