Actually, I think this model has thick hair enabling her to have such a dense, large, spiraling bun that's actually done on the back of her head and not higher. There are props that can be used under thinner hair such as a "hair roll or rat" that would be appropriate for this style. Hair not so thick can still be spiraled, it just wouldn't be so weighted as you all have guessed and it would be hollow in the center.
I've read where people have actually used nylons (pantyhose) rolled up and wrapped their hair around it for fuller looks to their formal styles. Since these are pliable and come in a variety of colors, they can easily be used under hairstyles with hair pinned to assure coverage. Movie stars from decades ago did this all the time. This model could have easily disguised one under her style, who knows?
Buns have been known to have foundations under them for years. Anything from things that look like filter baskets to coffeemakers to chignon doughnuts or better known as a hair rat. You can check out your local beauty supply store and see what hair forms they have.
Here are some interesting links for those of you who love shopping online.
Spiral Hair Stix
Hairgami Kit
Hair Rats and Rolls