The close and delete buttons appear to be padded with more space than the other buttons (share, edit, flag, protect). This is probably due to the new "close (1/5)" text that started appearing today.
I'm using Firefox 19 on a Mac.
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It appears to be a fault of the HTML itself, rather than the CSS.
<a id="close-question-15421502" title="vote to close this question (when closed, no new answers can be added)"> close </a>
Because of the extra lines within the anchor tag, it collapses them as spaces. The styling on the link is exactly the same as the other links otherwise.
Removing these extra lines fixes the problem.