You can run du to get a report on disk usage. If you set the -max-depth to 1 and run from the root folder, then it will tell you what directory is using the most space.

 $ cd /
 $ du -h -max-depth=1

For example, on my system the /var folder is the largest. So you can then repeat the process to find an oversized folder.

 $ cd /var
 $ du -h -max-depth=1

As for backups, I really hope they are not stored on the droplet. That kind of defeats the purpose of backups.

I guess -max-depth isn’t a possible argument on Ubuntu 14.04?

@Sinclair sorry, it should be --max-depth=1 with 2 dashes.

May not be relevant, but I also found my Digital Ocean droplet disk space filling up. It turned out to be old versions of the linux kernels - I used sudo apt-get autoremove to free up space. You can read my blog post on the subject free disk space on digital ocean droplets by removing unused ubuntu versions