Why does my photos app (with an empty library) take up almost 3 GB on my iPhone?

I’ve been trying to free up as much space on my phone as possible, and since I’m really not a fan of the way iCloud stores photos I’ve opted for a third party photo cloud storage option. Since I have backed up all of my photos safely I’ve deleted them from my photos app, and thus from iCloud. I have also turned off iCloud photo syncing. Despite all of this, my system preferences still shows that my Photos app is taking up about 3 gigabytes of documents and data storage on my phone. Why is this, and how can I reduce that? (I’ve already deleted all of my messages attachments also, btw.)