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., to either the devices SD card, a cloud account, a computer, or all of the above. Better to have copies away from the phone, that way you don't lose anything if the phone gets damaged, lost or stolen.
Then, clean up your apps. Remember just because you aren't actively using an app, doesn't mean it isn't sitting on your phone using up the phones storage and could be running things in the background, using up your resources, as well. Uninstall any apps you just aren't really using. You can always reinstall them again. Then on the apps you are keeping, check the individual apps to see if you can clear them up. Obviously you don't want to do that with ones that you need the info on. However when you go into "Settings" and then "Apps" and finally "each individual app", you will see the option to "Clear Data" and/or "Clear Cache". Make sure you only do that for apps you aren't really using much, or that you don't need the info for. You don't want to delete the data for apps you use regularly or games you play.