Currently all the statistics are processed and nothing is pruned. However this is not necessary to keep all the statistics. In phpMyVisites we used a simple technique that worked well, we kept only the first N elements for each data (N ~ 300 for example).
For example we keep only the first 300 keywords. We keep the first 150 URLs referers.
This will lead to a big time and disk space saving for huge websites.
Implemented and available in latest 0.2.3 version