I have a Naish Hokua 8'5" x 28" volume of 115.

Yesterday I tried a Naish 8'10" x 27.5" volume of 107.

I found the 8'10" to be much more nimble than expected and more so than my 8'5".

What is it that makes it so? Is it the volume difference? The half inch narrower? Or something else?

(By nimble I mean responsive etc).

So many things.. but mostly tail rocker and v IMO.

As well as DavidJhon said , can be center of flotation , with less flotation and better located reaction such between your feet while surfing.
For example Hokua8'5" should have flotation and reaction in front to secure surfriding. Instead of 8'10" getting its planning area under your weight situated more back of the board to make it loose but can be insecure riding...

HI
You are comparing two different models of boards, the 8'10" is the Hokua LE Pro performance as opposed to the normal Hokua surf range.
By Naish's own description: The LE Pro Performance boards are, 'ultra light weight, they feature an aggressive rocker from tip-to-tail.'
So I would agree with DJ, the rocker should make this very manouvreable but should need a little more push from the wave to get it going due to the lower volume and increased rocker.

Very hard to pick out just a couple of things, I'd have to include rail profile (hard or soft, where it transcends from soft to hard), bottom profile (concave nose if any, vee tail if any, where it transcends from concave to vee if any). I have weigh those equally along with the elements stated above, to decipher what kind of nimbleness that it will have.
And I didn't even mention fins, or how many.
It's not an easy question to answer w/o writing a dissertation, IMO.

Its always rocker first second and third. The other tricky one is the designed centre of balance of your board too far forward and it increases swing weight, this slows down the turn response of your board.

Apart from the board's design, different fins can have a big affect.