Well, then it’s probably being used, so increase it even more 
Mine is at 64MB and I never saw the message again. Note that the optimal opcache values depend on many factors, and there is no one-size-fits-all solution. You may have to experiment a bit to find the optimal settings for your instance.
Also, this has been discussed here many times, and I doubt anyone can add anything that hasn’t already been said. So please try the search, or the docs if you want to read more, or maybe start with the following thread: https://help.nextcloud.com/t/opcache-memory-consumption-insane-values-needed-by-nextcloud