For some reason, Sailfish OS on a Gemini PDA has a very small / but an overly large /home/.

This post describes how to reclaim space from /home/.

If you intend to build native graphical applications for the Jolla phone, then by all means follow the official docs.

If, OTOH, you are used to writing spec files and feeding another build system (for me it’s mainly Koji), then read on.