How did World War I make Hungary more ethnically homogeneous?

Following World War I, Hungary became more ethnically homogeneous primarily due to the massive territorial losses mandated by the Treaty of Trianon in 1920.1

Before the war, Hungary was a multiethnic state. However, the treaty stripped the country of approximately two-thirds of its territory and two-thirds of its inhabitants. Large regions previously under the Hungarian crown—including Transylvania, Slovakia, and parts of the Banat—were annexed by successor states such as Romania, Czechoslovakia, and Yugoslavia.1234

Because these ceded territories contained significant non-Hungarian populations, their removal from the Hungarian state left the remaining country with a population that was overwhelmingly ethnic Hungarian. Consequently, while millions of ethnic Hungarians were left as minorities in neighboring countries, the post-war borders resulted in a nation where more than four-fifths of the population identified as ethnically Hungarian and spoke the Hungarian language as their mother tongue.13