Yes. It is possible to have more space allocated for personal use.
First, I would suggest that your download Terminal Emulator and run the command
df in it to get the memory allocated for each partition (ie
The space allocated to
/data partition is only space you can use. Rest is for system/firmware that you cant use. There are three ways to get out of this problem->
We will reduce the amount of memory allocated to
/system and give that memory to
/data. Manufacturers often give
/system partition a farily good amount of space for future os upgrades. But when you use custom roms, they are relatively lighter and use much less space, so we can shrink this partition. But unfortunately, there isn't any such method made yet for xperia z ultra as far as I know. So this is a rather difficult (almost impossible, considering working of sony phones, the TA partition etc).
2) Installing as "system app"
You know that
/system partition has a lot of unused space. So how to use it? Install apps inside system partition! They will become system apps and you will need special app managers to uninstall them (you can use some app manager to install app and move it to
/system partition. Not all apps will support doing this). Downside is that apps will still store their data in
/data partition, so again, this isnt a very good option. (You can also just create a random folder inside
/system partition and probably store a few things like songs inside that? Ok that might be a stupid idea, even though I do practice this on my Nexus 4, which has relatively even less memory).
This leaves you with only one efficient option as this. Use some app like
Link2Sd to move apps and their data to external sd card. Hence more space inside internal. Again, not all apps are supported, but most are.