Matching the font size in standalone with IEEEtran

I have a tex file in IEEEtran with its default font size, which is 8pt. I want to include an image PDF, generated by standalone with font size 12pt. I set the scaling to be scale=0.666667, which is 2/3, to change the font size of the image to match with the main text. However, what I see at the end is that the text in the image is much smaller than 8pt. Why?
For example, the image is generated by

documentclass[12pt]{standalone}
usepackage{tikz}
begin{document}
begin{tikzpicture}
node [draw, align=center] {This is12 ptbefore scaling};
end{tikzpicture}
end{document}

and the main file is

documentclass{IEEEtran}
usepackage{tikz}
begin{document}
This is a sample text to show the default font size
in IEEEtran document class, which is 8pt.

includegraphics[scale=0.666667]{image.pdf}
end{document}

documentclass[class={IEEEtran}]{standalone}
usepackage{tikz}
begin{document}
begin{tikzpicture}
node [draw, align=center] {This isthe same ptbefore scaling};
end{tikzpicture}
end{document}