You should have the default Samsung file manager on your phone. It is called "My Files", if not you should be able to download it from the Galaxy Store.
Once you have it, long press on each folder and select "more" at the bottom and then "details". It will tell you the combined storage used by files in that folder. Then you can go through the individual files and see which are largest.
And upgrade your Micro SD card. 16gb is miniscule.