WP Statistics uses too much space

Hi there,
I really like your plugin but is there any way to limit the size of the tables within wordpress? Or can I delete some of old unnecessary data (not everything). The WP statistics tables have become too big and thats a severe problem if you only have limited space available for your database.
https://www.dropbox.com/s/w7uyinrhuhyqadw/Bild%204.png?dl=0
I purged data from older than 300 days but it didn’t help at all. The amount of entries decreased but the size of wp_statistics_visitor even increased by 10 MB…! Please let me know whats wrong.

Thanks in advance

Best regards