Are you shure?
Please check whit Titanium Backup maybe you coukd have 3 gb for data and only 512 for rom system (default partition)
I did't also have to change partition..
Everything is working well and so far no bugs on Miui6.
Except there's no Sim Toolkit and when i restore it with Titanium Backup, it installs, but when should be working gives "unfortunately the process com.google.process.phone has stopped" error...