Sounds like slightly more is used in /system than I'd expect, but entirely possible (I haven't used this particular ROM). You could try removing some apps from /system that you don't need.
It is possible to adjust the size of /system, have a look at the updater.sh in the zip file. Problem is that if you increase the /system partition by 500MB then you're decreasing the size of your /data partition by 500MB as well Note that this will also wipe /data.
/* * Your probably long dead warranty is now void. * * I am not responsible for bricked devices, dead SD cards, * thermonuclear war, or you getting fired because the alarm app failed. Please * do some research if you have any concerns about features included in this ROM * before flashing it! YOU are choosing to make these modifications, and if * you point the finger at me for messing up your device, I will laugh at you. */
lowmemorykiller: Killing 'system:ui' (5642), adj 902,