title: Size of PDF files generated by knitr
body: <p>I am using the following chunk in an <code>Rnw</code> file.</p>
  <pre><code>&lt;&lt;scatter,dev='pdf',include=TRUE&gt;&gt;=
  a &lt;- rnorm(1e4)
  b &lt;- rnorm(1e4)
  plot(a,b)
  @
  </code></pre>
  <p>I knitted the same <code>Rnw</code> file under <code>R3.6.3</code> and <code>R4.0.2</code>, respectively. However, the PDF figure generated under <code>R4.0.2</code> is much larger (~8 times) than the one from <code>R3.6.3</code>.</p>
  <p>I am wondering if there is a way (eg. a chunk option) to reduce the PDF size under <code>R4.0.2</code>.</p>
  <p>The session information of two different versions of R is shown below.</p>
  <pre><code>R version 4.0.2 (2020-06-22)
  Platform: x86_64-w64-mingw32/x64 (64-bit)
  Running under: Windows 10 x64 (build 16299)
  
  Matrix products: default
  
  locale:
  [1] LC_COLLATE=English_Australia.1252  LC_CTYPE=English_Australia.1252   
  [3] LC_MONETARY=English_Australia.1252 LC_NUMERIC=C                      
  [5] LC_TIME=English_Australia.1252    
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] knitr_1.29
  
  loaded via a namespace (and not attached):
  [1] compiler_4.0.2 magrittr_1.5   tools_4.0.2    stringi_1.4.6  highr_0.8      stringr_1.4.0 
  [7] xfun_0.16      evaluate_0.14
  </code></pre>
  <pre><code>R version 3.6.3 (2020-02-29)
  Platform: x86_64-w64-mingw32/x64 (64-bit)
  Running under: Windows 10 x64 (build 16299)
  
  Matrix products: default
  
  Random number generation:
   RNG:     Mersenne-Twister 
   Normal:  Inversion 
   Sample:  Rounding 
   
  locale:
  [1] LC_COLLATE=English_Australia.1252  LC_CTYPE=English_Australia.1252   
  [3] LC_MONETARY=English_Australia.1252 LC_NUMERIC=C                      
  [5] LC_TIME=English_Australia.1252    
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] knitr_1.28
  
  loaded via a namespace (and not attached):
  [1] compiler_3.6.3 magrittr_1.5   tools_3.6.3    stringi_1.4.6  highr_0.8      stringr_1.4.0 
  [7] xfun_0.12      evaluate_0.14 
  </code></pre>