If you select a diagram in the WebPortal and click zoom, it will be increased/decreased by the factor of 2. You can change the size of zoom as follows.
Open the file heading.xsl located in C:\Program Files (x86)\process4biz\Extensions\Publisher Server\ in the edit mode.
Find “zoom” and change the highlighted values 2 and 0.5 to 1.4 and 0.8 respectively.
Save the file.
Generate and publish website again.
Diagrams will now be increased/decreased by the factor that you have put in the file.
Alternatively you can edit the same file directly on published website (without having to republish the website again). Note that after the next publish, these settings will be reset to default.