ReiserFS (v3) is a great small filesystem that's fantastic at lots of small files (and also copes great with power outage events, like on my laptop for the last 15 years). I've had tons of issues with ext3/4 (running out of extents, slow performance on lots of small files), btrfs (running out of metadata space when I still have hundreds of GB left?!), xfs (great on everything except lots of tiny files or power loss). It even supported reliable shrinking and growing on LVM.
It's too bad no one is supporting it anymore, since the founder is in prison and the only other people who seem to be able to support it seem to be focused on a Reiser4 pipe dream instead of supporting great, reliable technology that had most of the bugs worked out a long time ago.