After the upgrade to Plesk 11.5 logs are not rotated.
The transition to the new business model after the upgrade to Plesk 11.5 was not completed.
Complete the transition to the new business model:
Tools & Settings > Panel > Complete Switching to the New Business Model
Run the DailyMaintenance Task to trigger log rotation procedure:
"%plesk_bin%\php.exe" -dauto_prepend_file="" -ddisplay_errors=on "%plesk_dir%\admin\plib\DailyMaintainance\script.php"