My diagram is currently large and I would like to reduce it.

```
\documentclass[a4paper,12p]{report}
\usepackage{tikz}
\usepackage{graphicx}
\begin{document}
\begin{figure}
    \begin{center}
        \begin{tikzpicture}[rounded corners,ultra thick]
        circles
        \shade[ball color=green] (0,0) circle (.5cm);
        \shade[ball color=blue] (-4,4) circle (.5cm);
        \shade[ball color=yellow] (0,4) circle (.5cm);
        \shade[ball color=red] (4,4) circle (.5cm);
        \shade[ball color=cyan] (-2,-4) circle (.5cm);
        \shade[ball color=orange] (2,-4) circle (.5cm);
        \draw (0,0) node[minimum size=2cm] {$A_e$};
        \draw (0,4) node[minimum size=2cm] {$I_m$};
        \draw (4,4) node[minimum size=2cm] {$I_s$};
        \draw (-4,4) node[minimum size=2cm] {$S$};
        \draw (-2,-4) node[minimum size=2cm] {$P_s$};
        \draw (2,-4) node[minimum size=2cm] {$P_I$};

    \draw[->] (-5.5,4) -- (-4.5,4);
    \draw[->] (-3.5,4) -- (-0.5,4);
    \draw[->] (0.5,4) -- (3.5,4);
    \draw[->] (-4,4.5) -- (-4,5.5);
    \draw[->] (0,4.5) -- (0,6);
    \draw[->] (4.2,4.4) -- (4.8,5.5);
    \draw[->] (4.5,4) -- (5.5,4);
    \draw[->] (0.4,0) -- (1.3,0);
    \draw[->] (1,-1.5) -- (0,-0.4);
    \draw[->] (-3.5,-4) -- (-2.5,-4);
    \draw[->] (-1.8,-4.4) -- (-3,-5);
    \draw[->] (-1.5,-4) -- (1.5,-4);
    \draw[->] (2.5,-4) -- (3.5,-4); 
    \draw[<-, dashed] (-3.8,3.5) -- (2,-3.5);
    \draw[->, dashed] (-0.3,-0.4) -- (-1.8,-3.7);
    \node at (-5,4.3) {$\pi$};
    \node at (-4.1,6) {$\mu S$};
    \node at (-2.5,4.3) {$\beta_h P_IS$};
    \node at (5,5.8) {$\delta I_s$};
    \node at (5.9,4) {$\mu I_s$};
    \node at (1.8,0) {$\phi A_e$};
    \node at (0,6.5) {$\mu I_m$};
    \node at (2,4.3) {$\sigma I_m$};
    \node at (1.5,-2) {$\epsilon P_I$};
    \node at (-3,-3.6) {$\theta P_N$};
    \node at (-3.5,-5.3) {$\theta Ps$};
    \node at (-0.2,-3.6) {$\beta_p(P_I+\gamma A_e)P_s$};
    \node at (2.9,-3.6) {$\theta P_I$};
    \end{tikzpicture}
    \caption{Model Diagram for HPAI dynamics}\label{fig:1}
\end{center}
\end{figure}
\end{document}

```