I didn´t get it.

There is a lot of symbolism in this story, and the message needs to be considered other than literally. His three parents respective nationalities represent the agricultural traditions of old Europe in conflict with industrial development. His decision to stop growing is timed with the early rise of the Nazis, and his decision to start growing again with his migration to the west. Oskar is the 20th century European, torn between cherishing an idealized rural past and a cosmopolitan future, who's growth is spitefully stunted by the war.
It is easier to gather this from to book, only about 2/3 of which were made into the film. There are some wonderfully poetic language and images which were left out, but then we would have had a six-hour movie. One thing that stands out for me is his educational reading material, pages torn from a trashy biography of Rasputin and Goethe's "Elective Affinities", the dark Dionysian and the bright Apollonian, form and chaos.
I'm really not intellectually disciplined enough, or arrogant, to claim that I completely understand the film or the author's intent, but I have always loved this movie for it's artistic value, for the impressions that it has left on my consciousness. I would recommend reading the book, and then trying the movie again--it's a very rich dish that deserves repeated tasting.