Where is my storage?

[Yo Dude1]

I have a Galaxy S4 for a second device. I have it rooted and once had a custom rom on it but took it off to avoid toubles with my carrier. Now i use it just on wifi and i notice after a full factory reset 6Gb is used by system and 6 Gb is used by me even though i factory reset it. I know at one point i could ise almost 12Gb of my 16Gb phone. Did i leave the cistom rom somewhere? How can i get more storage without an Sd card?

 

[B. Diddy]

I moved this from the main S4 forum to the S4 Rooting/ROMs forum for more specific traffic.