The evil villain Brainiac, used his Hyper-Ray to reduce the city and it's millions of inhabitants to a tiny size, and placed the city in a bottle. Years later, Superman was shrunk but found the machine Brainiac used to shrink Kandor (and other cities from other planets) and used it to restore the cities to normal. Unfortunately, at the end there was only enough energy to restore more thing to normal size--Kandor or Superman. A Kandorian decided Earth needed Superman more than Kandor needed to be normal size again and used it on Superman.
The city of Kandor is now kept safe in Superman's Fortress of Solitude as he tries to find a means to restore it ot normal size.
Season 4 (1979):