It depends somewhat on how you maintain the water. If you use Dichlor-only then yes having a larger spa volume has the water last longer because the amount of Dichlor needed for regular heavy bather load (i.e. soaking every day) is related to that bather load and not to the water volume, but the rate of Cyanuric Acid (CYA) concentration buildup is faster in a smaller volume of water. So the chlorine becomes less effective due to the higher CYA sooner in a smaller tub.
If you were to use the Dichlor-then-bleach method to avoid the CYA buildup, then the buildup is about organic waste that isn't fully oxidized and to a buildup of salt. So here again a larger tub has the water last longer, but you'll be at least double the time compared to Dichlor-only so the spa volume difference may not matter so much.
The other factor is that in a larger volume of water the chlorine you add after a soak won't have the FC rise as high. So there will be less wasted chlorine from outgassing. Also, any errors in dosing don't result in as much change in water values.
So yes, overall, a larger volume of water can be easier to maintain.
