Hungary lost two thirds of its land after World War I. Large parts of Romania, all of Slovakia, parts of Serbia and even a small piece of Austria as well as most importantly for Jews the Ruthenia/SubCarpathian-Rus area which includes Munkatch, Ungvar and other very large populations of Chareidim and Chasidim which became part of the newly formed Czechoslovakia, all were part of Hungary for many centuries until the end of WWI.
The Yidden from those former parts of Hungary spoke Hungarian and considered themselves part of Hungarian Jewry.