请问为什么我这个看起来不是a4大小

\documentclass[11pt, a4paper,oneside,landscape,openany]{ctexart}
\usepackage[a4paper, margin=2cm]{geometry}
\usepackage{lipsum}
\usepackage[many]{tcolorbox}
\tcbuselibrary{breakable}
\tcbuselibrary{skins}
\usepackage{lastpage}
\usepackage{amsmath, amsthm, amssymb, graphicx}
\usepackage[bookmarks=true, colorlinks, citecolor=blue, linkcolor=black]{hyperref}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{}
\chead{Notes:Mathematical Contest Handout for College Students(Pu)}
\cfoot{\thepage}
\newtcolorbox{answerbox}
{
colback=gray!20, colframe=gray!60, sharp corners,
leftrule=3pt, rightrule=0pt, toprule=0pt, bottomrule=0pt,{breakable},
left=2pt, right=2pt, top=3pt, bottom=3pt
}
\newtcolorbox{questionbox}[1]
{
colback=gray!10,
colframe=gray!80,
width=17cm,
arc=0mm,
title=#1,{breakable},
breakable,
enhanced jigsaw,
fonttitle=\bfseries,
left=15pt
}
%\label{1}
%\setcounter{equation}{0}
%\begin{figure}[htbp]
%\centering
%\includegraphics[width=8cm]{图片.jpg}
%\caption{图片标题}
%\begin{sloppypar}
%this is
%\end{sloppypar}
%\end{figure}
\begin{document}
\section{\Large Function,Limit,Continuous}
\subsection{Function}
\subsubsection{Comprehensive substitution}
\end{decument}
请问为什么我这个看起来不是a4大小