Have you tried any of the women's helmets? For the universal sized ones, they typically run smaller. Also, are you trying the universal sized helmets or the helmets that run S,M,L,XL? I know some of the universals are quite big. Normally though you have to step up in price to get the better sizing.

Also, when shopping for helmets, shop around and find one that fits really well. I don't even notice mine on my head once it's on. Some companies also offer crash replacement warranty, which is nice. I know Garneau offers one year crash replacement, basically you crash in the first year, bring it to your shop and they'll let you get a new one. Giro offers something like 30% off in the first year if you crash. It may be something to consider if you're doing more aggresive riding, but of course, fit is most important!

Lastly, for the larger head girls I read recently that the Bell Kinghead is the largest helmet on the market. For people with round heads, typically of asian descent, there is an Italian brand, Selev, that has made a helmet geared toward this. I'm not sure if they're available in the US though