It isn't really a problem, so much as you have used up the space and probably need to clean up the device.
Apps themselves take up a lot of space, as do photos, videos, music files, etc. Add to that the fact that some apps will also add even more data when you use them and save the information to the device. That takes up even more space. Next add any data from your phones browser(s). If you leave everything there, never cleaning the cache, etc., then that takes up more space.
First, I would suggest cleaning up your browser. Save any of your log in IDs, passwords, etc., and be sure to save sites you wish to keep in your favs. Then clear up your browser (browsers if you use more than one).
Next, copy your music, photos, videos, etc...