\rlap in subscript leads to larger subscript

Question

MWE:

\documentclass{article}
\usepackage{xcolor}
\begin{document}
$M_{\theta}$ % correct size theta
$M_{\rlap{\ensuremath{\theta}}}$ % too large theta
\end{document}

Why does this happen? How can I fix it? In my case the \rlap is inisede a command that I would like to work inside and outside of supscripts and superscripts:

\documentclass{article}
\usepackage{xcolor}
\newcommand{\rlaptheta}{\rlap{\ensuremath{\theta}}} % I would like the command to work both in normal mode as well as in subscript or superscruit
\begin{document}
$M_{\theta}$ % correct size theta
$M_{\rlap{\ensuremath{\theta}}}$ % too large theta
$M_{\rlaptheta}$ % too large theta
$\theta\rlaptheta$ %correct size
%The same for superscripts
$M^{\theta}$ % correct size theta
$M^{\rlaptheta}$ % too large theta
\end{document}