
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{lmodern}
\pagestyle{empty}
\begin{document}
Au premier ordre~:
\[
X_{AB} = X_{\scriptstyle AB} 
    \neq X_{\scriptscriptstyle AB}.
\]

Au second ordre~:
\[
X_{AB_{CD}} 
    = X_{AB_{\scriptscriptstyle CD}} 
    \neq X_{AB_{\scriptstyle CD}.
\]
\end{document}
