Btrfs not warning you are out of space?

I had chromium run in a BTRFS 4GB size container. It was showing I was using something like 3.25GB out of 3.55GB allocated. Then I move the container to a larger container (8GB) and all of a sudden I am using 4.11 GB out of 4.88GB allocated.

What in the world is going on? The smaller container felt running sluggish a little so I am suspecting I was running out of space. But how would I know? Is there a way to know BTRFS is running out of space? (Even if that means something BTRFS related, like “Hey, I cannot allocate more chunks!”)

UPD: it could’ve been the snapshot backups screwing the size data in some way after I moved the installation between containers. But the question remains about how to know when BTRFS cannot allocate more space.