Storage [Q] 1GB less after installing ROM


I recently installed the AEONFLEX rom on my 16GB D802, coming from stock rom.
Before flashing aeonflex, I had more than 10GB free on my storage, now I only have 9GB free after fresh install.

So is it the aeonflex rom that's taking 1GB of space ? Isn't it supposed to be installed on the system partition instead of the local storage ?