I want to use a big product symbol I have a problem

Question

\documentclass[14pt]{memoir}
\usepackage{lmodern}
\usepackage{amsfonts}
\usepackage{amstext}
\usepackage{amsmath}
\usepackage{amsmath}
\usepackage{mathrsfs}
\usepackage{amsthm}
\usepackage{mathtools}

\begin{document}
$$P:=\prod_{\sigma\in \textnormal{Gal}(L/K)}\big(X-\sigma(b)\big)\in A[X],$$
\end{document}