See also this question: http://stackoverflow.com/questions/8544333/mathjax-scale-options-for-inline-block
Contrary to the use case in the stackoverflow question I usually want the same behaviour as in regular LaTeX, so that inline formulas have the same font size as surrounding text but display formulas are bigger than the rest.
I may be misunderstanding you, but it sounds like you are asking for the font size for displayed equations to be larger than the font size for in-line equations. That is not the case for LaTeX (at least not in any style file that I know of, though I suppose it could be arranged). For example, in the amsart document class the fonts are the same for inline and displayed equations.
What does differ between the two is the spacing rules so that displayed equations are allowed to use more vertical space (inline equations try to reduce that so that interline spacing will not be affected). That means that displayed equations don't have to reduce the font size for things like fractions as quickly as inline equations do. Perhaps this is the effect you are referring to? But MathJax already does that the same way as LaTeX, so I'm confused.
If you can provide an example LaTeX file that shows the effect you are after, I'll look into it.
Hmm you are absolutely right. In fact, I now know why I was confused and thought it was bigger. The font rendering in small sizes (13px) doesn't look as good in Firefox as in Adobe Reader. I made some screenshots:
Firefox HTML/CSS (cleartype off):
Firefox HTML/CSS (cleartype on):
Firefox MathML (cleartype off):
Firefox MathML (cleartype on):
Shots were made under Windows 7 with Firefox 15.0.1 and Adobe Reader 10.1.4.
Do we mark this abandoned? I could imagine that the SO question could be turned into an extension but I don't understand if this thread still has a question in it.
I think at this point the complaint is about the quality of the fonts, but this is largely due to the poor font rendering system in Windows. It is possible that there is something that can be done to the font (hinting or something) that might help, but I don't have the expertise to do that.
You are right that something could be done to make an extension that allows different sized display and inline math, but I it is low priority. I would mark it abandoned (or address later if you want to keep the idea for the extension around).
I'm ok with closing this as abandoned. It can always be re-opened or referenced in a new feature request.