so...

As an easter egg, the high-resolution (pixel-doubled) version of the comic is merely the comic resized to 50% and then to 400%, making it an image of poorer quality rather than a higher resolution image as for other comics, demonstrating how repeated image scaling can also introduce artifacts into images. There appears to be another easter egg in which between 21 November 2016 and 10 December 2016 (both Internet Archive pages), the original hi-res image was replaced by the pixel-doubled image. However, the image URL below the comic still links to the hi-res image.

The img-tag also includes the pixel-doubled version as a srcset-attribute: srcset="//imgs.xkcd.com/comics/digital_data_2x.png 2x" This causes browsers on high-resolution displays (pixel density ratio 2) to show the low-quality version of the image instead of the standard-quality one and leads to people re-visiting the comic on modern displays to experience degradation compared to the remembered version.

https://www.explainxkcd.com/wiki/index.php/1683