I'm guessing that most of that is due to thumbnails and/or database files for the videos and photos. You could use a file manager to go to the /DCIM/.thumbnails folder and see what happens if you delete all of them. But the system will probably just create more the next time you open your Gallery app, or browse the web. My Nexus 5's thumbnails folder is around 780 MB in size, and I rarely keep a lot of photos saved locally on my phone.
You have a lot of free storage, so it id doubtful those misc files will impact your device's performance.