Markdown images and <figure> images rendered differently

I've been migrating all my posts from a Squarespace site, and am running into something strange. Images derived from the Markdown ![Image caption.](image_url.png) format follow the 100% width formatting defined in the template CSS. But images derived from a raw HTML <figure> tag do not follow the same formatting, and I can't figure out why. Is there something I need to change or add to the CSS file to have them rendered the same?

Here's an example post where both formats are represented.