You might just want to go to a snowboard or bike shop, and try some on. Some fit very differently on your head. I've tried a Pro-Tec Ace Dig for biking (pinched the sides of my head), Pro-Tec Mercenary for snowboarding (perfect fit, but heavy), and a RED Hi-Fi (perfect fit, and light). All of them were the same "size" as well.
And you're also aware that a bike helmet on the slopes is going to freeze your head? I wanted to do the exact same thing as you, but I didn't want to sacrafice warmth. Ended up buying a B2 bike, and Hi-Fi snow.
Try them on in person!!