Can you post a screenshot.

Normally there should not be a lot of stuff. The old stats should be deleted automatically.

Here is a screenshot of the graph. The table shows every day the blog was online and got hits.
(This is my sandbox blog. startet Dec. 2014)

Ok, your hitlog is not pruning, go to the following settings as set your settings as shown:

http://b2evolution.net/man/hit-logging

Thanks a lot!
I had chosen "Prune old hits & sessions (includes OPTIMIZE)" in Scheduler and at auto pruning by job. This did not work. But putting auto pruning on every day the problem could be fixed.