Standard posters are 25cm x 70cm and exist with four different motives. To prepare them for a specific concert a sticker with the respective information is printed and put on the poster. Printing the posters in 50cmx70cm is really cheap (they are black and white - i.e. only "one color") if you order around 1000 copies. Cutting is for free, so essentially per concert the stickers are more expensive than the posters. Still a very reasonable way to get cheap posters in a decent and unusual size.