I just purchased this fabulous Swedish children's book (luckily translated into English) from Etsy seller sorrythankyou79.
The book was written in 1964 by Hans Peterson, published by Ab Rabén and Sjögren Bokförlag with illustrations by Ylva Källström. It tells the story of an old man and his pet bird. When the bird becomes ill, the man gives it a pill which cures the bird but makes it grow and grow until it is as big as a circus tent (and still growing). The story is as delightful as the pictures.