Only 2KB minified & gzipped (core). Each language definition adds roughly 300-500 bytes.
The language-xxxx class is inherited. This means that if multiple code snippets have the same language, you can just define it once,in one of their common ancestors.
Very easy to extend without modifying the code, due to Prism’s plugin architecture. Multiple hooks are scattered throughout the source.
Very easy to define new languages. The only thing you need is a good understanding of regular expressions.
All styling is done through CSS, with sensible class names rather than ugly, namespaced, abbreviated nonsense.
Wide browser support: Edge, IE11, Firefox, Chrome, Safari, Opera, most mobile browsers.
Highlights inline code as well, not just code blocks.
It doesn’t force you to use any Prism-specific markup, not even a Prism-specific class name, only standard markup you should be using anyway. So, you can just try it for a while, remove it if you don’t like it and leave no traces behind.