Yeah I've always wondered, usually when I put my beanie under my helmet it looks pretty ridiculous. It could just be the beanie or would I want to buy a larger sized helmet in order to make the fit better?
if it looks wack, then you should buy a helmet thats a size up. Just get a thin and small beanie, then go to a shop and buy a helmet that you have tried on with that hat and you're goggles. I have an echeadwear "baller hat" it fits good.