Here is the stackoverflow question:

http://stackoverflow.com/questions/9304192/emacs-linum-mode-and-size-of-font-unreadable-line-numbers

Here is another question with a hacky solution

http://unix.stackexchange.com/questions/29786/font-size-issues-with-emacs-in-linum-mode/30087#30087

I can repro this with recent builds of Emacs24

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Le