I have a book with the font size of equation smaller than that of text. Please look at how small $\pi$
compared to the rest:
\documentclass[openany]{book}
\usepackage{amsbsy}
\usepackage{changepage}
\usepackage{geometry}
\geometry{paperheight=9.8125in,paperwidth=8in, left=.5in,
right=.5in,top=.75in,bottom=.4375in }
\usepackage{fancyhdr}
\fancypagestyle{plain}{\fancyhf{} \fancyhead[EL,OR]{\textbf
{\thepage}}\renewcommand{\headrulewidth}{0pt}}
\pagestyle{plain}
\renewcommand{\rmdefault}{pag}
\usepackage[utf8]{inputenc}
\usepackage[hyphens]{url}
\usepackage[authoryear,round]{natbib}
\newcommand{\titlefontarash}{\usefont{OT1}{pag}{b}{n}\fontsize{25}
{\baselineskip} \selectfont}
\newcommand{\basefontarash}{\usefont{OT1}{pag}{b}{n}\fontsize{18}
{\baselineskip} \selectfont}
\newcommand{\datefontarash}{\usefont{OT1}{pag}{m}{n}\fontsize{12}
{\baselineskip} \selectfont}
\begin{document}
\title{\titlefontarash respect}
\author{\basefontarash good boy}
\date{\datefontarash April 21, 2019}
{
\maketitle
\let\cleardoublepage\relax
\frontmatter
}
\vspace*{\fill}
\begin{center}
\basefontarash To beauty...
\linebreak
\linebreak
\basefontarash To nice...
\end{center}
\vspace*{\fill}
\tableofcontents
\begin{adjustwidth}{2.25in}{0pt}
\mainmatter
{Introduction}
{\chapter{beauty}
fish (\citet{fox(1994)}) number $\pi +\pi +\pi +v\pi +\pi +v\pi +\pi +\pi
+\pi +\pi +v\pi +v\pi +\pi +\pi +\pi +\pi +\pi +\pi +\pi +\pi +$ fish
\textbf{RIVER ${\boldsymbol{\pi+\pi+\pi+\pi+\pi+\pi+\pi+\pi+\pi}}$}
following .
\section{fox1}
\subsection{fox2}
\subsection{fox3}
\subsection{fox4}
\subsection{fox5}
\subsection{fox6}
\subsection{fox7}
Now we are fox...
Let us see what happens
\section{fish}
fish fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog
catvfish fox dog catvfish fox dog catfish fox dog catfish fox dog catfish
fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog
catfish fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox
dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog catfish
fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog
catfish fox dog cat
}
\section{fish5}
fish fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog
catvvfish fox dog catfish fox dog catfish fox dog catfish fox dog catfish
fox dog cat
\section{dog}
fish fox dog catvvvfish fox dog catfish fox dog catfish fox dog catfish fox
dog catfish fox dog cat
\subsection{dog ${\boldsymbol{\pi}}$}
\section{mouse}
\section{milk}
\chapter{pishi}
\end{adjustwidth}
\begin{thebibliography}{9}
\bibitem[fox(1994)]{fox(1994)}fox, T. L. (1994) \textit{fox.} New York: fox.
\end{thebibliography}
\end{document}
Do you know how to change the fontsize of equation for the entire book in one command line not to do it for every equation?