I just measured the outside at what looked to be the widest part, and it's almost 14 inches around. (This may be a little misleading since the leather is thick and stiff and it doesn't mold to your legs the way, say, Jimmy Choo high heeled boots do. So the circumference inside must be quite a bit smaller.) I've had these boots and worn them quite a bit since early October, so I am sure they have stretched. I don't wear thick things with them, only leggings, tights or very skinny jeans. I agree that they look narrow because they don't taper in a lot at the ankles the way more form-fitting boots do.
The only way to know for sure is to try them on with different things you want to wear with them and decide if you are comfortable.
My SA said that Hermes would stretch them for me if I want, but I decided to wait and see if I can stretch them myself just by wearing them around. I found one side to be a tiny bit difficult to take off myself, but it is improving each time I wear them.