Internal Storage Memory reduced after installing custom ROM I just installed Cyanogenmod 12 into my HTC Desire 816 , for flashing I used TWRP. So the whole proccess was as follows:


Install CM via TWRP
Reinstall CM and GAAPS


Everything works fine and dandy but I have one big problem. My internal storage only shows 4gb of total space, from which I have only like 1GB of free space available. The phone originally came with 8gb of internal storage, and is just after I did this that it was reduced. I did clean up all the Dalvik cache, I also tried doing a new factory reset, but still it's showing only 4gb.
Unfortunately, devices with small ROM are prone to quick "runout", as correctly pointed out by previous comment. It is indeed true that the "actual" ROM size is not the one stated from the vendor but the one dictated by the Android OS itself. As an example, try to insert an SD card to your computer then right click and get to the properties. You will see that although empty , the available memory is less than the total memory reported. This is because also the SD card host some hidden files necessary for the proper functioning of the card itself.
Hopefully you will have a slot for a microSD card on your phone so that you can save all your media and transfer most of the apps to it thus keeping your internal ROM as free as possible.