Hungary TRIED to switch to the Allies in 1943, sending several representatives undercover to the UK to discuss the issue. The idea was rejected immediately, and the meeting publicized, leading to the arrest of the representatives by Germany upon their return to the mainland. Germany occupied Hungary a few months later to forestall any further attempts, so a change in 1944-45 was not possible with the country effectively reduced to puppet status and physically occupied by German troops. After the war, Hungary had no representation in the process, either on the Soviet or Allied side, so the matter was totally out of their hands.
Majorities in the disputed areas are a highly debatable issue, because many things were done by virtually all parties to cloud the waters. In many of the regions in question, there was no "majority", but numerous individual communities primarily of one ethnicity or another in a patchwork that made any simple dividing line "wrong". In other words, on the Czechoslovakian border there were Magyar (ethnic Hungarian) towns, Slovak towns, German towns, and so on. On the Romanian side, in most areas there again was no "majority", but Romanians had a plurality in many, with individual villages and cities being primarily Romanian, Magyar, German, Serbian, or other. I recall one breakdown showing over 12 different ethnic groups in one county on the Yugoslavian border, none comprising over 20% of the total population. The relocation of families and changing of family names in a couple of countries, to confuse the issue and break up the ethnic solidity of several of those communities, only muddied things further.
Note that during my visit to Hungary in 2002, we passed one small town which was ethnically practically 100% Italian, settled by hired Italian artisans during the late 1800s. The community used Italian as its primary language (Hungarian was taught as a second language), had Italian customs, and was effectively a piece of Italy stuck inside of Hungary, with the main exception being that all correspondence with the national government was required to be in Hungarian. People tended to settle in cultural enclaves, and when the Magyars, Germans, Romanians, Serbs, and other groups settled areas such as Transylvania, they did so in villages primarily or entirely of their own ethnic groups. You can't draw a nice straight line to separate the black and white squares on a chessboard, yet that is what the victors of the World Wars attempted to do, for various expedient reasons, and the entire Balkan region has paid the consequences.
As said in another post, there was no "motivation" for the Soviets or the Allies to allow Hungary to retain the border areas it regained with the First Vienna Awards.
