So... i wanted to download this app "ZenUI marshmallow theme" it was 2 megabytes. It prompted me - there isn't enough space to install this app. 207MB needed. YES! TWO HUNDRED FREAKING MEGABYTES! It's 100 times the size! Why i this happening?!
Because memory is used for cache and data of all apps on the phome not just for installing apps, on most phones when free memory hit number of ~500 mb they show message 'insufficient-memory' when try to install some. I doubt that theme need 200mb, no theme require that much space.