User blog comment:SupremeAssassin/Assassin's Creed IV: Black Flag - Epilogue and Thoughts/@comment-5174994-20140110174005/@comment-4928711-20140113175639

That doesn't mean it's a longer game. For example a memory in Assassin's Creed III could take 6 minutes to complete, where as a memory in Assassin's Creed IV Black Flag could take  24 minutes. So it's not just about how many memories but how long they are.