On JSF there has been some recent discussion in one of the comment threads about the mass return of nationalities deported by Stalin to their historic homelands in the late 1950s and early 1960s as a precedent for the return of the Palestinian refugees. The question of scale has been raised. I believe there are about 4 million Palestinian refugees and their descendants living outside of historic Palestine. This is certainly greater than the total number of deportees in the USSR that returned to the Caucasus, Kalmykia and other regions during the Khrushchev years. But, the numbers involved were still quite sizable.
Between 1941-1948, the Stalin regime forcibly deported 3,266,340 people from their homelands to eastern regions of the USSR (Bugai, doc. 48, pp. 264-265). A total of 2,303,279 of these exiles came from the eight nationalities deported in their entirety (Ediev, table 109, p. 302). That is the Russian-Germans, Karachais, Kalmyks, Chechens, Ingush, Balkars, Crimean Tatars and Meskhetian Turks. By 1 January 1954, the total number of special settlers had declined to 2,760,471 (Bugai, doc. 62, pp. 277-278). The largest group of deportees, the Russian-Germans were never allowed to return to the Volga, Ukraine and other European areas of the Soviet Union in large numbers. The Crimean Tatars and Meskhetian Turks also remained banned from returning to their homelands during most of the Soviet period.
The Chechens and Ingush were the largest group to return home from exile in the eastern regions of the USSR. Between 1957 and 1961, a total of 384,000 Chechens and 84,000 Ingush returned to the North Caucasus leaving only 34,000 Chechens and 22,000 Ingush in Kazakhstan and Kyrgyzstan. The vast majority of these returnees, 432,000, returned to the Chechen and Ingush ASSR. This increased the territory's population to 892,400. The remaining 36,000 returned to their ancestral villages in Daghestan and North Ossetia (Bugai, doc. 66, pp. 281-282). Thus nearly 90% of the Chechen and Ingush population had returned from exile in Central Asia to the North Caucasus in less than five years.
So I do not have an example of millions of refugees or deportees returning home. But, there is an example of hundreds of thousands of internal exiles in the USSR returning home during Khrushchev's reign. Of course one major difference between Israel and the USSR is that the return of the Chechens and Ingush to a restored Chechen-Ingush ASSR did not threaten the existence of the USSR as a socialist state. The return of an equal percentage of Palestinian refugees to their ancestral homeland would undoubtedly doom Israel as a Zionist project.
N.F. Bugai, ed., Iosif Stalin - Lavrentiiu Berii: "Ikh nado deportirovat'": Dokumenty, fakty, kommentarii (Moscow: "Druzhba narodov, 1992).
D.M. Ediev, Demograficheskie poteri deportirovannykh narodov SSSR (Stavropol': "Argus", 2003).