Question :: "This brine keeps overflowing! I put the batch in a larger jar and submerged it was another jar of water like you mentioned, but the brine keeps spilling out. How does volume increase like this? Just expanding? Oh wait, I pushed down on it, and it released a ton of bubbles. So I was able to pour that brine back in, and it seems like I'll just keep doing that? Should I put it in divided jars?"
Answer :: You are right that pressing it down and letting the brine sink back into the cabbage is what needs to be done. The jar is quite full and so there isn't much room to accommodate the action. You will need to press it down regularly, or you can press it into a larger jar. Thanks for the photo!
As far as the CO2 action you are seeing--it is different all the time. Some cabbage batches are not as active as well…its a funny thing and hard to predict. The good news is even the quiet ones are working.