The thing is, about waveriding (DTL which in most eyes is the only thing counting) in light wind, that many only has a 9 or 10m2 as their biggest size.
Then some dont ride in lighter wind, where these sizes are not sufficient.
Also, in many countries/spots, waves do not build much if lower wind (not ground swell).
The major difference between say a 10 and a 12m2 in marginal winds, is:
The 12 might be able to get you going upwind, where the 10 will not...
BUT, you can still ride waves a lot better with the 10 than the 12, and it will generate at least the same power, maybe even more because it turns way better and more efficient when going DTL - and drifts better too.
So even when too light wind to go upwind with the 10, many takes a downwinder on the 10 because it is much better and more fun than riding with a 12m2
Last but not least, some ride with a 12m2 so possible to stay and go upwind (at some spots the only way) - this is the most difficult way of riding (some will say it does not work also, and somewhat correct), but can be fun and challenging sometimes too.
Not many does this though, but as said, can be fun now and then too.
So there are many reasons why 9-10m2 is the typical max size used in waves (for the average rider weight)
Having said that - I sometimes ride with bigger kites in waves too myself and find it okay fun, but this is definitely not the "norm" around the world, thats for sure.
As some mentioned - a bigger board is a great way to do it !