http://www.telegraph.co.uk/technology/news/10843541/Cannes-Film-Festival-how-do-they-make-those-giant-posters.html
Köck added that, what has really made it possible to print the massive posters is the evolution of digital printing. 15 years ago, many billboard posters were still produced using offset printing, which was very cost effective if you were printing 10,000 copies of the same poster but became very expensive for short print runs.
Digital printing makes it much more cost-effective to print a small number of posters, or even unique ones. It even makes it possible to print personalised fliers or labels, as in the case of Coca Cola's recent campaign, whereby it created personalised labels with people's names on.