Pokémon Sun/Moon did not outsell Pokémon X/Y during the Japanese launch, however, it still did quite good in terms of sales, coming in as the second best-selling 3DS game at launch in Japan, as reported by Famitsu.
The games sold 1,905,107 copies at retail as of today, but this number doesn't include the download versions which Nintendo keep quiet about. Pokémon X/Y sold 2,096 million units in the first four days excluding digital, and "only" 866,570 copies in Japan during the first two days. It also sold another 473,151 copies in its second week for a total of 2,339,721 during the first nine days, also excluding digital copies.
Did you expect Sun/Moon to be this successful?