Not sure if there is anything peculiar about the android implementation by Sony. Though to be fair, 16gb is already small in the first place, by today's standards

For me, on all my phones, usually it's apps data
Google earth seems to take up/store a lot of offline data
Also Maps, of course
Other apps would be whatsapp and other messaging services that has to store the photos n videos, although nowadays i see that whatsapp can already store them in the external card.
How about your camera? Make sure it stores the video/pics in the sd card also

This post has been edited by abu.shofwan: Apr 7 2017, 01:11 PM