First of all I know exactly how you feel. I thought my baby had a normal sized head until we went for our 6 week check at the doctors. She measured and said that we'd need to keep his head circumference under observation as it was on the 98th percentile. I was really upset as thought this meant that there was something wrong. Also, my cousin had hydrocephalus (sp?) so asked her if it could be that and she said no because his fontanelle had no rigid parts and was normal. Still, it bothered me and I phoned up the health visitor.
When the health visitor came round she was great. It turns out that my baby's weight, height and head circ are all around the 95th percentile, so in proportion. She then monitored for a month or so and left it.
So, I think if there was something wrong it would have been picked up. Like someone else said her lack of hair is making it look larger (my son is the same). It's also the case that the more you stare at something the bigger it then gets in your mind (like when you have a spot as a teenager - ha ha!). The other thing I was told was that another symptom of hydrocepthalus was very poor head control and I can see in the pictures that this is not an issue for you little one.
I would definitely mention it to your HV as I think she will be able to put your mind at rest. Like you, I don't think your MIL meant to upset you.
P.S: Bella is gorgeous!