I think many casinos perform SOW checks more often on smaller players to have some evidence that they are doing something. But they prefer to leave those bigger players alone since they generate the most profit.
Here's an example with me. I used to play on Sky and never had to go through SOW until one day, after depositing around 14-15K. One of them called me and asked if I knew how much I had already pumped in today...
But, of course, they only called when the money was gone, not after the first or second deposits. Because they expected that I would deposit more since i was depositing 2k at a time.
Before, I used to play at their site in the same manner, but nobody gave 2fcks. By the way, Sky is the unluckiest casino i have ever played. I'm not complaining about my actual monetary losses, but such bad luck with games is simply unreal. It actually seems like some of their games are specifically built for their site.
There are also 2 more casinos i know that would happily take my and all my neighbours' money without checking SOW. But i won't name them for now.