I had problems in a diagram when using Call Activities because they are not resized properly (see my attachment). The selectable border is resized, but neither the bold box of the task nor the label containing the text are resized accordingly. As a workaround I messed around with the values in the .activiti file until it had a somewhat appealing look…
While searching the forums I found this topic related to this JIRA bug, so apparently this issue already has been fixed for other components but not for the Call Activities.