title: Size of font in ggplot plot changes in relation to plot using knitr
body: <p>I'm using knitr for the first time, and have a problem concering the font size in ggplot plots. This is an example plot:</p>
  
  <pre><code>d &lt;- ggplot(diamonds, aes(x = cut, y = clarity))
  d + stat_sum(aes(label=..n..),geom="text",size=8)
  </code></pre>
  
  <p>In knitr I have the same plot in a chunk in my R markdown:</p>
  
  <pre><code>---
  title: "Untitled"
  output: html_document
  ---
  ```{r, echo=FALSE}
  library(ggplot2)
  d &lt;- ggplot(diamonds, aes(x = cut, y = clarity))
  d + stat_sum(aes(label=..n..),geom="text",size=8)
  ```
  </code></pre>
  
  <p>The plot looks fine in RStudio or when saved with ggsave(). However the numbers in the plot in the resulting knitr html have a much larger font size, in total and relative to the plot size:</p>
  
  <p><a href="https://i.stack.imgur.com/KNDaO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KNDaO.png" alt="ggsave image"></a> <a href="https://i.stack.imgur.com/sgOO7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/sgOO7.png" alt="knitr image from html"></a></p>
  
  <p>In this example it does not matter much, but in my data the numbers start to overlap each other / run out of their cells.</p>
  
  <p>An added complication is that the plot is done by a package, so I can't easily change the size option in the stat_sum call.</p>