I have now opened the file myself in a sqlite editor, and executed:
SELECT COUNT(*), URL FROM URLs GROUP BY URL HAVING count(*)>2 ORDER BY 1 ASC, URL;
That showed the reason for the huge size...
In the results I saw 2 entries are very often in this File:
5245153 http://192.168.0.1:49000/upnp/control/WANIPConn1
30229249 http://192.168.0.1:49000/upnp/control/WANCommonIFC1
That means the major reason for this big size, is that avast stores millions of scanns from upnp connections to my router.
The question now is, how i could stop Avast from dooing this ?