Jean Dujardin was shortened with different camera tricks like being shot from above, on his knees or sitting on a wheeled stool. During his dialogue scenes with Virginie Efira, which were often shot without these tricks, he had to look up (thus above her head) while she had to look down (at his chest).
Alexandre played by Jean Dujardin says he is 136 cm in the film, though in real life he's 182 cm.