You're most probably right. I run my system on a 20gb btrfs partition and I have run into issues before where I ran out of space because of one big binary file that was in every snapshot.
That shouldn't be an issue. Btrfs snapshots are copy-on-write, so unless all blocks changed the big binary file should be there just once.
Yeah.. It happened to be a qcow2 image for a virtual machine. I meant to put it on another disk but accidentally forgot to set the right path.