% with fix-cm
% ecbx1200 at 24.88pt
% sfbx1200.pfb
%\fontname\font
\sffamily
% without fix-cm
% ecsx2488
% sfsx2488.pfb
% with fix-cm
% ecsx1000 at 24.88pt
% sfsx1000.pfb
\fontname\font
\end{document}
The documentation says
"Only those design sizes of the fonts will be used, that are
normally available in Type1 format, too. You need not load the
extra package cmmib57 for this purpose."
I do find the reference to an old (and now obsolete) package like
cmmib57 (http://mirrors.ctan.org/fonts/amsfonts/doc/cmmib57.pdf)
quite dubious. And with the cm-super fonts there are certainly more
design sizes available than used by the sty.
I do understand that is could be problematic to change fix-cm as it
could change existing definitions. But couldn't one create a
new-fix-cm.sty with better font definitions?