Reduced font size of equations in align environment

Question

When I use 'align environment' to write equations, the displayed equations (eq 1.5 & 1.6) look smaller in font size compared to other equations (eq 1.7 & 1.8). For reference I attached screenshot of the compiled document. How to make sure all equations are of same font size?

\begin{align}
\pd{f}{x} &= \pd{f}{z} \cdot \pd{z}{x} = \pd{f}{z} \cdot 1 \\
\pd{f}{y} &= \pd{f}{z} \cdot \pd{z}{y} = \pd{f}{z} \cdot i
\end{align}

since $f(x) = u(x,y) + iv(x,y)$ by \autoref{eq:complexfunction}, we also have

\begin{equation}
\pd{f}{x} = \pd{u}{x} + i \pd{v}{x} \quad \text{and} \quad \pd{f}{y} = \pd{u}{y} + i \pd{v}{y}
\end{equation}
Combining above equations, we have

\begin{equation}
\pd{f}{x} = \pd{f}{z} = \pd{u}{x} + i \pd{v}{x} \quad \text{and} \quad \pd{f}{x} = \pd{f}{z} = \pd{u}{x} + i \pd{v}{x}
\end{equation}

preamble (if required)

\documentclass[a4paper]{article}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\usepackage{jheppub}
\usepackage{amsthm}
\usepackage{tcolorbox}
\usepackage{commath}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\tcbuselibrary{theorems}
\newtcbtheorem
  []% init options
  {theorem}% name
  {Theorem}% title
  {%
    colback=green!5,
    colframe=green!35!black,
    fonttitle=\bfseries,
  }% options
  {thm}% prefix

\newtcbtheorem
  []% init options
  {definition}% name
  {Definition}% title
  {%
    colback=red!5,
    colframe=red!50!black,
    fonttitle=\bfseries,
  }% options
  {def}% prefix

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\newcommand{\reals}{\mathbb{R}}
\renewcommand{\qedsymbol}{$\blacksquare$}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\subheader{Notes}
\title{Some Title}
\author{Some Author}
\affiliation{Greatest University}
\emailAdd{[email protected]}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}
\maketitle
\flushbottom
\newpage

Answer 1

A more reasonable test document is

\documentclass{article}

\usepackage{amsmath}
\usepackage{commath}
\begin{document}

align
\begin{align}
\pd{f}{x} &= \pd{f}{z} \cdot \pd{z}{x} = \pd{f}{z} \cdot 1 \\
\pd{f}{y} &= \pd{f}{z} \cdot \pd{z}{y} = \pd{f}{z} \cdot i
\end{align}

equation
\begin{equation}
\pd{f}{x} = \pd{u}{x} + i \pd{v}{x} \quad \text{and} \quad \pd{f}{y} = \pd{u}{y} + i \pd{v}{y}
\end{equation}
\end{document}

The commath package is based on a misunderstanding of \ifinner and is forcing a text style setting in align The package really can not be recommended, use a simple \frac rather than commath version of \pd.

\documentclass{article}

\usepackage{amsmath}
%\usepackage{commath}
\newcommand\pd[2]{\frac{\partial#1}{\partial#2}}
\begin{document}

align
\begin{align}
\pd{f}{x} &= \pd{f}{z} \cdot \pd{z}{x} = \pd{f}{z} \cdot 1 \\
\pd{f}{y} &= \pd{f}{z} \cdot \pd{z}{y} = \pd{f}{z} \cdot i
\end{align}

equation
\begin{equation}
\pd{f}{x} = \pd{u}{x} + i \pd{v}{x} \quad \text{and} \quad \pd{f}{y} = \pd{u}{y} + i \pd{v}{y}
\end{equation}
\end{document}