Hendrik,
What version did you go from/to?
Tweaking your Client Data retention as Venu mentioned is the key to reducing your RRD sizes.
it also depends how many transient users your Airwave is seeing.
For example, we have an Airwave server with just north of 2400 APs monitoring a large coffee chain serving up public Wi-Fi.
Our AMP server (8.0.8.1) has 1.5Tb of disk space and currently using 81% of it and RRD's are taking up more than 60%
Our Client Data Retention is set to 14 days so we don't run out of disk space