Math-mode font size increase for document global font size above 14pt

Question

I'm writing a memoir with a \documentclass[b4paper,14pt,english,twoside]{memoir} and was very surprised that math formulas are so small.

Then my goal is to increase math-mode font size but keep a 14pt document class.

Totally working for 12pt

I've searched a workaround, here with the following MWE (and amplified font size increase)

Here I set a 12pt document class and increasing math mode font size with \DeclareMathSize is working:

\documentclass[b4paper,12pt,english,twoside]{memoir}
\usepackage[english]{babel}
\usepackage[utf8x]{inputenc} % For French accents
\usepackage[T1]{fontenc}
\usepackage{lmodern, textcomp}




%%%%%%%%%%%%%%%% MATH et TiKZ %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage{amsmath}
\usepackage{amssymb}

\DeclareMathSizes{12}{20}{14}{10}

\begin{document}
test old $\displaystyle \lambda$
    \begin{equation}
        \displaystyle a\ge b_h \iff \sum_{j:a_j\ge b_h^j}
    \end{equation}

\end{document}

Which is totally working for 12pt memoir:

Doesn't work anymore for 14pt

Trying to change global font size from 12pt to a 14pt and keeping an increased math mode font size doesn't work anymore.

\documentclass[b4paper,14pt,english,twoside]{memoir}
\usepackage[english]{babel}
\usepackage[utf8x]{inputenc} % For French accents
\usepackage[T1]{fontenc}
\usepackage{lmodern, textcomp}




%%%%%%%%%%%%%%%% MATH et TiKZ %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage{amsmath}
\usepackage{amssymb}

\DeclareMathSizes{14}{20}{14}{10}

\begin{document}
test old $\displaystyle \lambda$
    \begin{equation}
        \displaystyle a\ge b_h \iff \sum_{j:a_j\ge b_h^j}
    \end{equation}

\end{document}

\DeclareMathSizes doesn't work anymore, switching to a 14pt document class:

When it's working and when it's not

After few other tests, it seems that the command is working for any nn pt if nn <= 12pt and it doesn't work anymore for any nn pt if nn >= 14pt (with nn being one of the allowed sizes for memoir class).

Another test ran by @daleif showed that it's not working neither for extarticle document class which also provides 14pt and 17t font sizes.

Why doesn't it work anymore, when above 14pt and what solutions do I have to fix it?

Answer 1

Life is never easy. You need to know the exact font size. memoir and other classes that provides an 14pt size class option (e.g. extarticle), is actually not using 14pt font size, the use \@xivpt, which is actually 14.4!! And 17pt is 17.28.

This stems from the old days when the fonts a certain sizes had to be generated. Nowadays where we have scalable fonts, things are a bit blurry, so things like this gets overlooked.

You can check the value using

\makeatletter
\typeout{\@xivpt}
\makeatother

Then

\DeclareMathSizes{14.4}....

seems to work just fine.

Perhaps better

\makeatletter
\DeclareMathSizes{\@xivpt}....
\makeatother

The \@... macros being used can be found in the mem<size>.clo files that comes with memoir