I'm undecided as yet whether 7th to 8th or 8th to 9th is the smallest.
Neither felt like a normal sized generational leap to me, but it's a bit early to judge the latter as there haven't really been any big 9th gen AAA exclusives yet.
Bet with Liquidlaser: I say PS5 and Xbox Series will sell more than 56 million combined by the end of 2023. (And over 130 million lifetime)