Quality over quantity. D:OS1 felt really long to me, but it felt padded with tons of inventory management and lame fights that took forever.
Another thing to consider is that D:OS2 will probably have way better replay value than D:OS1, since there's going to be so many different possible results for things, and also you have 6 origin stories but only 4 members. Also consider arena fights, GM mode, and a better platform for mods as a way to vastly increase total playtime in an interesting way.
That said, the first island did feel pretty quick to me, though I surprisingly got 14 hours. I probably did 75% of the stuff, perhaps less. The overall flow and distribution was pretty solid. I usually felt like I was accomplishing something. Part of the problem is the lack of challenge, as well as knowing it will end, so I didn't feel like I needed to seek out every power boost I could get. Harder fights and more of a need to build good characters will definitely add a couple hours.
All of these factors will add to the playtime without padding the game with bad quests or trash fights, as many games do to increase playtime.