I never noticed a lot of size difference between mountain and road helmets, but I have noticed a size difference between manufacturers.
I've seen an advertisement from Limar saying that their helmets are smaller.

It seems to me that most of the mountain helmets have a removable visor and road helmets don't have a visor.

I ride only road bikes but all my helmets are mountain helmets. I want the visor to keep as much of the sun off my face as possible. My doctor says I'm prone to skin cancer but that doesn't keep me from riding. I'll take my chances with the sun rather than not ride and die a slow death from all the problems related to being overweight.

Don't tell anyone but I ride without the visor a lot of the time.

My new (3 months old) helmet (Giro Animas) has lots of vents (23) to keep my head cool in the summer and the RocLoc3 system to keep it securily on my head.