"He thought that size was really good for two little children's hands, and he loved the visual impact, too," longtime friend Marja Kerkhof, of his Dutch publisher Mercis, said in a telephone interview.
Bruna "passed away peacefully in his sleep" Thursday night in the central Dutch city of Utrecht, Kerkhof said.
The simplicity of Bruna's characters drew adoration not only from children, but also from adult art lovers. Amsterdam's venerable Rijksmuseum put on a show featuring his work in 2015.
He wrote and illustrated a total of 124 books, but Miffy, known in the Netherlands as Nijntje, a contraction of the Dutch word for rabbit, was far and away his most popular and best known character.
Bruna created 32 books about the rabbit, which were translated into more than 50 languages and sold more than 85 million copies, Kerkhof said. The man his publisher described as Miffy's "spiritual father" stopped drawing in 2011.
Miffy, who turns 62 this year, is a merchandising juggernaut, featuring on stationery, toys and children's trinkets sold across the world as part of a multimillion-euro (dollar) business.
"He was very much loved around the world. I remember traveling with him to Australia, to New Zealand, to Asia, to Japan. Wherever he would go, people would queue up for signing sessions of his books," Kerkhof said.
She said the public's affection for both Bruna and Miffy stemmed from the illustrations' simplicity.
"It is very clear pictures, almost like a pictogram, and the fact that he leaves so much out — he goes to the essence of things — and of course his very strong, powerful primary colors," she said. "Even today, if you see it in a store you would think, 'hey this looks different to a lot of other things out there.' There is no clutter, it's all very clear."
A museum in Utrecht dedicated to Miffy tweeted: "Today the sad news reached us that Dick Bruna has died." The message appeared above a simple but emotionally powerful black-and-white drawing of the famous rabbit, arms behind her back and a single tear below her left eye.
Utrecht municipality said flags on city hall would fly at half-mast Friday and Saturday as a sign of respect for one of the city's most famous sons.
The 2015 show at Amsterdam's Rijksmuseum put Bruna in his historical context, showing his drawings and book covers — he was the son of a Dutch publisher — alongside artists who influenced him like Henri Matisse and Fernand Leger.
Curator Caro Verbeek said the simplicity of Miffy was an illusion. Verbeek said she had other curators at the museum draw Miffy's head, with its dots for eyes and "X'' shape representing both nose and mouth.
"They did not succeed," she said. "It is incredibly difficult. But the fact that it seems so simple proves that he is a true genius."
Funeral arrangements were not immediately announced.