How to free up space by installing cm on my note2 topic

I noticed that when I am on stock rom, android takes more than 5gb of space out of 16 GB and I am left with around 10 GB. But when I install Cyanogenmod which is around about 300mb, I still get around 10 GB of free space out of 16. Why is that?
If I am removing a bigger rom and installing a compact one in place of it, shouldn't I be getting more space?