Lenovo p780 usable internal memory lowered to 3GB from 6gb

Sorry about my grammar. I have lenovo p780 8GB row.So I was using the miui rom for a long time and thought of flashing the AOSP 4.4.4 row rom(by bauu).started by doing a full twrp backup.went to recovery mode while wipeing cache.davilik and data I accidently wiped system and the rom failed to instal.so I flashed back to stock jelly bean.rooted the phone and forwarded to instal the rom wiped cache davilik data and installed the rom the rom installed wiped cache and davelik.phone started without any problems.but the rom was using only external memory for storing apps and stuff and had some issues with camera and decided to go back to miui.went to recovery wiped cache davelik data and installed different version of miui(latest version)I was using v.5 or something before.usually I get 5 to 5.80 GB of free space in any roms I tried but I had only 3 Gb this time :( so flashed back to stock.tried factory reset and clearing cache in stock recovery. But couldn't recover the lost memory.operating system occupied 5GB space :eek: . I have no idea what to do any HELP is greatly appreciated
thanks in advance

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