In xfce4-panel 4.9+ icons have size that fits a single row. Clipman plugin should follow this convention. Currently the icon stretches to the full panel size, which make it stick out among all other icons.
Created attachment 4145
A patch implementing this feature
Should be compatible with earlier versions of the Xfce panel.
Fixed in git master f205871..5c2e72b
Thanks for your patch.