Avoid small tag in `align` environment wrapped around `small`

Question

I am working in an align environment, where I have a long equation. To make it fit within the line width, I wrap a small environment around the align one:

\begin{small}
\begin{align}
...
\end{align}
\end{small}

The problem I have is that not only is the size of the equation reduced, but also the size of the tag number at the end of the line, a feature I want to avoid.

How to have a math environment in small font size except for the equation tag?

Answer 1

The equation tag is typeset using \normalfont which invokes the eponymous hook. You can just add \normalsize to that hook to also reset the font size.

This requires a fairly new LaTeX since the hook management was only introduced recently.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{small}
  \begin{align}
    ... \tag{small tag}
  \end{align}
\end{small}

\begin{small}
  \AddToHook{normalfont}[normalsize]{\normalsize}
  \begin{align}
    ... \tag{normal tag}
  \end{align}
  \RemoveFromHook{normalfont}[normalsize]
\end{small}

\end{document}