Are you sure you don't mean the First World War? Changes to Hungarian borders after the second conflict have pretty much only restored their pre-war territory, as you can see on this map. Whatever parts of their neighboring countries the Hungarians have occupied during the Second World War mostly returned to their previous owners.

Hungary's territorial gains are tied to the two Vienna Awards mediated by Germany and Italy in 1938 and 1940, and a result of the Axis invasion of Yugoslavia in 1941.

The First Vienna Award was a consequence of the Munich Agreement, and saw Hungary receive parts parts of southern Slovakia and Transcarpathian Ruthenia, with further gains in Ruthenia following the German occupation of Czechoslovakia in 1939. The Second Award resulted in Hungary getting a large part of until then Romanian Transylvania. In Yugoslavia, Hungarians moved into of the Bačka region with little resistance some time after the initial German attack.

However, as the end of the war drew nearer, Hungary would inevitably lose these territories. Ruthenia was ostensibly liberated by the Red Army in 1944, however, the Soviets would soon take over and the Czechs officially ceded the region to the Soviet Union after the war. In the same year, partisan and Soviet forces liberated Bačka and it returned to Yugoslavian control. As the war ended, Hungary were basically back to their original borders, with the Paris Peace Treaty further underlining this with its annullment of the Vienna Awards.

But if it is indeed the changes after the first War that you are interested in, look up the Treaty of Trianon, which saw areas with non-Hungarian majority populations break away from the Kingdom of Hungary on the basis of the principle of self determination, and also paved way for Hungarian irredentism which led to the above expansion during the second War.