Hungary in 1918 was 125,500 sq. mi.
Croatia, which they absurdly acquired after being defeated by the Hapsburgs, is 22,000 sq. mi.
Transylvania, the birthplace of Romania which is absurdly given to the Kingdom of Hungary after 1867, is 40,000 sq. mi.
These two territories you say they shouldn't have gotten back in 1919. So when people claim that Hungary lost 70% of 'historic Hungary', 50% of that 70% are those two territories. Two territories they unjustly received after being defeated by the Croatians and Romanians in the 1848 war.
In addition to those, the Hungarians lost the aforementioned (Serb plurality) Vojvodina, which is 8,300 sq. mi. They also lost the Ukrainian majority territory of Transcarpathia to Czechoslovakia (5,000 sq. mi). This region voted for union with Czechoslovakia, so I'm wondering what the problem was there. Upper Hungary, which you claim is not "so-called" Upper Hungary but an actual legitimate term, comprises most of current day Slovakia, and is 20,000 sq. mi.
If you're keeping track, all of that adds up to 75% of 'historic Hungary'.
So somebody tell me what injustice was done here. None of these areas were Hungarian majority areas. And in at least one of them (Carpathia) there was a plebliscite. In two others (Vojvodina & Transylvania), bloody wars were fought against Hungarian rule.
Looks like justice to me. The 1940 borders gave Hungary a huge number of Slovak speaking areas and effectively closed off Slovak transportation links with Europe. Their annexations in Ukraine took the best farmland from Transcarpathia. Even after Hungarian annexations in the Vienna awards in the late 1930s, Hungary still, even against German wishes, moved to take more Slovak speaking regions.
So sorry, they were led by nationalists, they didn't have just grievances.