Back hoofs smaller than front?

So I have a 7yo thoroughbred gelding, who never raced. He has front shoes but no back ones because we feel its not necessary. Well tonight someone pointed out that his back hoofs are only like .5cm at the back and out at the front. We looked at other horses in the barn and theres werent to large... and his skin doesn't touch the ground, but i was wondering if this is normal and what to do? the farrier is coming on friday