Knitr ioslides：自定义代码块大小

Question

As per this answer和其他资源，我的理解是在为Rmarkdown演示文稿设置CSS时，code.r {}应该控制回显代码的参数，而pre {}应该控制输出的大小。但是，pre {}似乎正在改变它们和code.r {}什么都不做。我做错了吗？

---
title: "CSStest"
author: "Me"
date: "December 27, 2017"
output: ioslides_presentation
---
<style type="text/css">

body{ /* Normal  */
   font-size: 16px;
}
code.r{ /* Code block */
  font-size: 4px;
}
pre {
  font-size: 42px
}
</style>

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE)
```

---
### Slide with R Output
```{r echo = TRUE}
##here is some R code
1+3

##Why is the output the same size as the code?
```

我在Windows上使用Pandoc 2.0.5，rmarkdown_1.8.5和knitr_1.15.1。

编辑：这是我屏幕加载幻灯片的html：

<slide class=""><hgroup><h2></h2></hgroup><article >

<h3>Slide with R Output</h3>

<pre class = 'prettyprint lang-r'>##here is some R code
1+3</pre>

<pre >## [1] 4</pre>

<pre class = 'prettyprint lang-r'>##Why is the output the same size as the code?</pre></article></slide>


  <slide class="backdrop"></slide>

</slides>

我还尝试了一种稍微不同的方法来自定义幻灯片，基于每张幻灯片，如this thread中所述。

---
title: "CSStest"
author: "Me"
date: "December 27, 2017"
output: ioslides_presentation
---
Presentation Title
========================================================
author: Jane Doe
transition: none

<style>
.small-code pre code {
  font-size: 1em;
}
</style>

Slide 1 - large code
========================================================
Regular slide text here

```{r}
head(cars)
```
Slide 2 - small code
========================================================
class: small-code

More regular slide text...
```{r}
head(cars)
```

但同样，字体大小没有变化

更新：我意识到有一个较新版本的Knitr，但是使用knitr_1.17我遇到了同样的问题。