A4 尺寸输出，带 a0poster；A0 尺寸仅与某些包结合使用

Question

这是我的代码：

\documentclass[a0,landscape]{a0poster}
\begin{document}
Hello, World!
\end{document}

当我编译它时，我得到 A4 大小的输出。

$ pdflatex foo.tex
This is pdfTeX, Version 3.14159265-2.6-1.40.19 (TeX Live 2018) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./foo.tex
LaTeX2e <2018-04-01> patch level 2
Babel <3.18> and hyphenation patterns for 22 language(s) loaded.
(texmf/tex/latex/a0poster/a0poster.cls
Document Class: a0poster 2004/01/31 v1.22b a0poster class (GK, MW)
(/usr/local/texlive/2018basic/texmf-dist/tex/latex/base/article.cls
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
(/usr/local/texlive/2018basic/texmf-dist/tex/latex/base/size10.clo))
(texmf/tex/latex/a0poster/a0size.sty)) (./foo.aux) [1
Non-PDF special ignored!
<special> header=./a0header.ps
Non-PDF special ignored!
<special> papersize=118.82cm,83.96cm
{texmf/fonts/map/pdftex/updmap/pdftex.map}] (./foo.aux) )</usr/local/texlive/20
18basic/texmf-dist/fonts/type1/public/amsfonts/cm/cmr17.pfb>
Output written on foo.pdf (1 page, 9922 bytes).
Transcript written on foo.log.

$ open foo.pdf # Shows 8.27 x 11.7 inches document

$ sips --getProperty all foo.pdf
/Users/lone/foo.pdf
  pixelWidth: 1240
  pixelHeight: 1753
  typeIdentifier: com.adobe.pdf
  format: pdf
  formatOptions: default
  dpiWidth: 150.000
  dpiHeight: 150.000
  samplesPerPixel: 4
  bitsPerSample: 8
  hasAlpha: yes
  space: RGB
  profile: sRGB IEC61966-2.1

$ echo "scale=2; 1240 / 150; 1753 / 150" | bc
8.26
11.68

即使请求的是 A0，但最终的输出却是 A4，我做错了什么？

顺便说一句，我注意到一件奇怪的事情，当我在序言中使用hyperref或graphicx或包时，我得到了所需的 A0 输出（46.78 x 33.06 英寸）。例如：xcolor

\documentclass[a0,landscape]{a0poster}
\usepackage{hyperref}
\begin{document}
Hello, World!
\end{document}

但导入其他包（例如multicol或）titlesec仍然会导致 A4 尺寸输出。这是怎么回事？