Light and modular
You can choose to include only the features that you need using the online build tool or by compiling it yourself with Grunt.js. Size of core JS file is about 3KB + each module weighs about 0.5KB (gzipped). Sass CSS preprocessor is used for easier skinning, but you're not obligated to use it.
Content is resized with CSS
The majority of lightbox plugins require you to define size of it via JS option. Magnific Popup does not - feel free to use relative units like EM's or resize lightbox with help of CSS media queries. Update content inside lightbox without worrying about how it'll resize and center.
High-DPI (Retina) display support
Default controls are made with pure CSS, without external graphics. For the main image there is a built in way to provide appropriate source for different pixel density displays.
Plugin has an option to automatically switch to alternative mobile-friendly source on small screen size. Brad Frost has a terrific article about this technique.
Popup has an extendable micro templating engine that reuses existing DOM elements (example), which is especially useful when your popups same pattern.