Android free space decreasing
As every time you use any app, their is some caches for each sessions which is saved by app in device memory like if you are using Instagram it stores some of your starting posts in your device memory as caches for offline view. So as you clear your cache every time, device clear all the temporarily saved data and when you starts again using that app, it again saves some cache for your easy handling of app in offline view. So that's why your memory is decreasing