My apologies, Bernd. I was thinking you were using the Jasper preview bean when I answered. We use the Jasper preview bean (Smart client only) with the following code:
elements.bean_jasper_preview.displayMode = plugins.jasperPluginRMI.JR_SVY_VIEWER_DISPLAY_MODE.FIT_PAGE
and, it works as expected. Give the Jasper preview bean a try!