If I'm not mistaken, I believe video shows as "other." Do you have a lot of video recorded? If not, download WinSCP, ssh into your phone and start checking directories' sizes.
Everytime you jailbreak there's a default root partition sized at 750mb (Disk Space) And Just Cross That 750mb Out, no worries. Also, your filesystem reserves space as well in order to work. If you get an out of the box iPhone that is 8Gb, It will not be 8Gb actually a little less. Sometimes memory runs out and tries to use virtual memory from your space, but that can't be it. That's basically it. Packages and 750mb of space unless you have lots of official appStore apps. Root partition at default differs from JB tool to tool.