Why aren't the arrows put under the equal sign if they are less than .7em or .3cm?
\documentclass{article}
\usepackage{mathtools}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}
\newcommand{\bigswarrow}[1]{\tikz{\draw[->](#1,3ex)--(0,0);}}
\begin{document}
\foreach \wid in {.1,.2,...,1}
{\bigswarrow{\wid em}}
\foreach \wid in {.1,.2,...,1}
{$\underset{\bigswarrow{\wid em}}{=}$}
\foreach \wid in {.1,.2,...,1}
{\bigswarrow{\wid}}
\foreach \wid in {.1,.2,...,1}
{$\underset{\bigswarrow{\wid}}{=}$}
\end{document}