Piwik works fine since october 2010 and my database is about 7Go (5Go before 1.2 update !). We use OpenX as adserver and it doesn’t shown any visitor decrease.
Are you able to compare with google analytics for a few days (or another service)? it is possible that it fixes a bug where it would previosuly overcount (we fixed a few similar to this in 1.2)
HellR I can see the difference, but I have no idea where to start really… do you have errors in your error logs? are you sure you tagged all your pages with both tags? do you use any third party plugins? also check: Troubleshooting - Analytics Platform - Matomo which contains a lot of info
I am experiencing the exact same problem my hits went from 3000 a day to 1000. I had been running piwik for a few years so the sudden drop is very noticeable on my graphs.