It looks like the menu plugin's title has a vertical cap on it, so that if the panel size is below 30, the title is not centered. With my font, the title is even partially hidden in the button at size 24, whereas taskbar 'buttons' still show the font fine.
Brian, this one is for you I believe. Just give it back if you don't want it ;-)
I don't see any problem here until I get below 20px on the panel size. EVen then, it's just because the button has more vertical padding than tasklist buttons. With the tasklist, I see the icons start to get cut off, and the text is no longer centered. I think you'll either just have to live with it, or use an rc file to set your font size smaller. Or, stop worrying about every single tiny pixel of desktop space and make your panel a little larger ^_~.