It's doing what it's supposed to do. It's not a bug - it's due to the content.

This is not an issue with page previews but the endpoint

The summary produced by the article is:

<p>At a resolution of 1440 x 900 or smaller popups should appear above the following links.\n<br id=\"mwAw\">\n<br id=\"mwBA\">\n<br id=\"mwBQ\">\n<br id=\"mwBg\">\n<br id=\"mwBw\">\n<br id=\"mwCA\">\n<br id=\"mwCQ\">\n<br id=\"mwCg\">\n<br id=\"mwCw\">\n<br id=\"mwDA\">\n<br id=\"mwDQ\">\n<br id=\"mwDg\">\n<br id=\"mwDw\">\n<br id=\"mwEA\">\n<br id=\"mwEQ\">\n<span>With image</span><br id=\"mwEw\">\n<span>PreviewsWithNoImage</span><br id=\"mwFQ\">\n<span>With tall image</span></p>

You'll notice the article has lots of <br> tags (line breaks)
Since they are explicit HTML they are not treated as whitespace so become part of the lead paragraph.

These br tags are followed by text.

We can of course remove those br tags, but they might be important in other contexts. I think this is very unlikely to occur in production so I would caution against adding complexity to the endpoint to deal with it. <br> tags may be necessary in other articles.

I should note, it's possible using css inside Page previews to make <br> tags invisible. However that will result in text being unintentionally squashed:

@Jdlrobson - good catch. I think we can close this.

So it is shrinking to fit its content but its content is whitespace. I don't mind resolving this as declined as long as we document the behaviour (br tags aren't stripped).

Done: https://www.mediawiki.org/w/index.php?title=User%3APhuedx_%28WMF%29%2FReading%2FWeb%2FPage_Preview_API&type=revision&diff=2641073&oldid=2586675