And no, this isn't about property sizes in Japan!
Bill Gates or Warren Buffet might be very surprised if they were to buy a Japanese manshon, only to find upon their arrival something a bit less grand than they envisioned, and that they would be sharing it with quite a few other people.
The English edition of Wiktionary claims that it is derived from English:
From English mansion.
Note that while English “mansion” most commonly means “large house”, it is also occasionally used to mean “luxurious apartment”, the latter usage being followed in Japanese.
And the English language Wiktionary article talks about mansion meaning "A luxurious flat" being UK English, which I've heard was the main source of English in the 19th century.
Japanese English - 10 Truly Bizarre Examples Of Wasei-Eigo claims it might be borrowed from French:
Don't get too excited if you're moving to Japan to live in a mansion. These mansions are actually condominiums. I have no idea how this creation came about. Some of the mansions I've seen in Japan were less than luxurious. Is it a joke? I suspect it might be a borrowed word from French.
Is 「マンション」 derived from English, or French? And did Japanese mess up the meaning of "mansion", or were they being faithful to a meaning that is rarely used nowadays?