Changing font size in equations changes superscript size in text (LATeX)

Question

I used \DeclareMathSizes{12}{20}{14}{10} to make my equations larger than the default output of LATeX (the parameters mean, as far as I understand, {font size}{equation regular text size}{equation superscript size}{supersuperscript size}). However, the superscripts outside the equation environment also became 14pt font! I thought that the \DeclareMathSizes command would only affect the content of my equations. Is there a way to change the superscript size in math mode without affecting the parameters for rest of my text?

EDIT: Here is a minimal example that reproduces my problem, note that changing the third parameter also influences what happens to the superscript outside the equation. Is this normal? How can I stop this from happening?

\documentclass[12pt,oneside,letterpaper,titlepage]{article}
\DeclareMathSizes{12}{20}{14}{10}

\begin{document}
...modulation by Ca\textsuperscript{2+}...

\begin{equation}
\sigma^2 = iI - \frac{I^2}{N}
\end{equation}
\end{document}

**SECOND EDIT/FINAL SOLUTION: I ended up following a suggestion to do something much simpler: I just used the \LARGE command on my equation. Here's the same example as above, using \LARGE instead of \DeclareMathSizes:

\documentclass[12pt,oneside,letterpaper,titlepage]{article}

\begin{document}
...modulation by Ca\textsuperscript{2+}...

\begin{equation}
\LARGE{\sigma^2 = iI - \frac{I^2}{N}}
\end{equation}
\end{document}

Answer 1

The Problem lies within the definition of \textsuperscript that uses math mode.

\DeclareRobustCommand*\textsuperscript[1]{%
  \@textsuperscript{\selectfont#1}}
\def\@textsuperscript#1{%
  {\m@th\ensuremath{^{\mbox{\fontsize\sf@size\z@#1}}}}}

A possible workaround is redefining the actual command so that it will not get in the way with your equations, this needs graphicx:

\renewcommand{\textsuperscript}[1]{\raisebox{0.8ex}{\scalebox{0.66}{#1}}}

Or this needs relsize

\renewcommand{\textsuperscript}[1]{\raisebox{0.8ex}{\smaller{#1}}}

I admit that this is just an easy workaround emulating the typrsetting of superscripts. It might be possible, that modern typography defines super/subscript in a certain way.

This also works with various enlargements of text sizes. This MWE also emulates a textsubscript:

\documentclass[12pt,oneside,letterpaper,titlepage]{article}
\DeclareMathSizes{12}{20}{14}{10}

%% Solution 1
\usepackage{relsize} %smaller
\renewcommand{\textsuperscript}[1]{\raisebox{0.8ex}{\smaller{#1}}}
\newcommand{\textsubscript}[1]{\raisebox{-0.4ex}{\smaller{#1}}}

%% Solution 2
%%\usepackage{graphicx} %scalebox
%%\renewcommand{\textsuperscript}[1]{\raisebox{0.8ex}{\scalebox{0.66}{#1}}}
%%\newcommand{\textsubscript}[1]{\raisebox{-0.4ex}{\scalebox{0.66}{#1}}}

\begin{document}
...modulation by Ca\textsuperscript{2+}...
...soluted in water H\textsubscript{2}O...\\
\tiny Ca\textsuperscript{2+}
\scriptsize Ca\textsuperscript{2+}
\footnotesize Ca\textsuperscript{2+}
\small Ca\textsuperscript{2+}
\normalsize Ca\textsuperscript{2+}
\large Ca\textsuperscript{2+}
\Large Ca\textsuperscript{2+}
\LARGE Ca\textsuperscript{2+}
\huge Ca\textsuperscript{2+}
\Huge Ca\textsuperscript{2+}
\normalsize

\begin{equation}
\sigma^2 = iI - \frac{I^2}{N}
\end{equation}
\end{document}

Further reading relsize and graphicx.

For chemistry typesetting (I guessed that on the calcium bit) there are some packages available, like bpchem or mhchem, that handle formulas quite well.