\documentclass[12pt,a4paper]{book}
\usepackage{amsmath,amssymb,amsthm,color}
\usepackage{xepersian}


%\newcommand{\BigFig}[1]{\parbox{0.1pt}{\large #1}}
%\newcommand{\BigZero}{\BigFig{0}}

\newcommand{\BigFig}[1]{\text{\Large #1 }}
\newcommand{\BigZero}{\BigFig{0}}

\begin{document}

$$x+\BigZero =y$$

\end{document}