From Sandeen's blog post, "The XFS filesystem has taken a beating for being a big, complicated, foreign filesystem since it’s introduction, and there is no doubt that there is a fair bit of code in there. But an interesting thing happened on the way to the Linux Kernel v3.0.0 - XFS developers have steadily reduced lines of code, while other up and coming filesystems such as Ext4 and BTRFS are steadily growing in LOC and complexity. And XFS has been under constant improvement at the same time as well."
Eric also notes that the code for the XFS file-system is more well documented than Btrfs/EXT4. There's also thousands of lines of duplicate code between these file-systems. It's not too surprising though that the Btrfs file-system size is on the rise as it's still under active development and new features are continuing to be added.