Although I don't know why some BB's head is quite big at the age of 6-7 months, but I know somehow they will become to normal size at the later age at about 9-10 months. This is the case found at my brother's daughter. She's now 11 months old. There is nothing wrong with her so far. Don't worry!! When you see the doctor next month, ask him in details. At the same time, I can ask my brother if there is any information for your reference.
One of my friends has a baby with very large head. Her baby had to conduct some of the checking when he was small and the result was normal. If you see his parents, you will not be surprised of his large head. Both of them have very large head. Now the baby is nearly three years old and everything is normal. Don't worry!