The IRC nick bit looks that way because there is a manual style="font-size: 120%" attribute in the page.
As for the bug pages, the text is at the same size as the size as the proportional text (both the font height and the ex-height look about the same to me). Perhaps it'd be worth making monospace text slightly smaller than proportional text though -- I believe that's what Firefox's defaults do.
The IRC nick bit looks that way because there is a manual style="font-size: 120%" attribute in the page.
As for the bug pages, the text is at the same size as the size as the proportional text (both the font height and the ex-height look about the same to me). Perhaps it'd be worth making monospace text slightly smaller than proportional text though -- I believe that's what Firefox's defaults do.