The premise of the question is incorrect.

Since the Russo-Japanese war in 1905 Japan had occupied the Southern portion of the island of Sikhalin (north of Hokkaido), which had been administered as a prefecture since 1907. In 1945 near the end of WWII when the Soviets ended their neutrality pact with Japan they also invaded South Sikhalin. The territory, along with several of the small Kuril islands between Hokaido and Kamchatka, were not returned to Japan after the war. The Japanese populations of these territories were largely repatriated before the 1950s, though there is an ongoing territorial dispute about the Kuril islands.

As for Italy, aside from areas in the Adriatic and Aegean seas that could best be characterized as "possessions", Italy did lose several long-held territories at the end of WWII. Along the border with France near the Mediterranean the cities of La Brigue and Tende were forced to be ceded to France, though these changes were upheld by local referenda. Tende, for example, had been part of Italy since Italian unification and the formation of the Kingdom of Italy in 1861.

Additionally, the city of Trieste along the Adriatic coast which had been a part of Italy since after WWI was held as a "free city" until the mid 1950s when it was split up and administration was given to Italy and Yugoslavia (and officially integrated as territories of those respective nations in the 1970s). A larger area called the "Venezia Giulia" or Julian March was also made part of Italy in 1920 after WWI and was ceded permanently to Yugoslavia after WWII.