Marking module has a font size setting.
However, that same font size is used for both the preview and the single view (= archive?) streams. Since both of these streams can -and probably normally have- different screen resolutions, the text markings end up showing with the right size on the single view and huge on the preview view.
Possible solution: Xeoma would rescale the markings when on preview pane taking into account differences in stream resolution.