I had the chance to demo a fanatic prowave @ 92L. Comparing to the Starboard Pro I had, even though it is 10L less and 1/2" narrower, I found the prowave more stable. Does it have a straighter outline than the Starboard? In any case, it seems volume plays little part in stability.
i think it depends a lot on where the volume is on the board more than simply the total litres, as well as rocker, deck shape, outline and rail thickness...
That is not a lot of volume difference if the conditions get lumpy and bumpy. If conditions are deteriorating more volume will make it all easier.
It depends a lot on your weight.
Basically if you weight 75kg, there will not be a lot of difference between a 92l board and a 102l board.
However, if you weight 90kg, the difference will be huge.
For me, Volume has two separate functions:
- a pure "Archimedes" flotation role. There is a difference if you sink or not. Quite a binary ("on/off") factor. Basically a volume/rider-weight ratio (the guild factor) relates to the stamina and technique the board demands.
- a kind of "raw material" to be used by the shaper to distribute in the board to enhance either stability (by spreading it to the rails and tips) or pivot (by centering it in domes or camel decks)
A board can be stable but tiring (sinking "Tomo") or unstable but paddlable for hours (narrow longboards)
People tend to look at volume for stability, but it is a mistake. Widths are.
Surfnbjj,
because lengths or model years were not stated in the question...
I used the Prowave and Stby PRO with the volume closest to what you listed to compare the boards.
Prowave 7'10 at 95 L and Stby PRO 7'7 at 107 L
if these are wrong, I apologize in advance.
I do board comparisons everyday when consulting SUPsters on their next board and I find that often numbers are just numbers.
There are a lot of factors that go into the comfort level on any board.
Surface area and width are key, I feel both are significantly more important than volume.
Board shape, rocker and bottom contours also play a significant role.
I always compare board options to the riders current board and factor in their comfort level on the board they are riding now.
I have ridden many brands and models, but I still really need to come up with ways to look for subtle differences.
One way is to use a graph I created to look at boards side by side and by laying the outline of one over the other
This is not always accurate because manufacturers usually use the smaller boards in the model line in all their pictures.
Boards size up with slight differences, so I am using pictures that may not represent the sizes I am comparing.... but its is a tool that I find useful as a starting point
I made up a chart of the boards you are asking about to see if I could see something that would explain your experience
Given these are both small sizes, I think the pics are pretty accurate for comparison
Probably easier to see on a computer, rather than a phone 
I feel that the higher volume of the Stby PRO would be a bit more corky than the lower volume Prowave, thus feel less stable
Wow, thanks for the detailed answers. Great food for thought. I must admit, I have fallen into the trap of thinking more volume = more stability in the past. This is good food for thought.
