Without reading the docs in depth, there's a difference of about 142KB of render-blocking CSS. I'm sure the API and functionality depth is very different too.

Well, I meant why should one use this over bootstrap.

Faster loading times on mobile come to mind.

People using Bootstrap end up serving a lot of CSS that isn't even used in the end. Using a minimal framework like that one allows you to have a solid skeleton without any extra features on which you can build.

It is 28 times smaller? (mini.css 7KB, Bootstrap 199 KB)

I'm trying to come up with a situation where loading a 199kb css file would make a difference. Maybe websites that target an audience that is expected to have a slow connection (banks etc.) but this kind of rules them out by not supporting IE.

Have you used a mobile connection in the subway lately? That's a key use-case, I think.

Mobile is especially important for page load.

Bootstrap requires JQuery.

If you use just the CSS part of it, you won't need jQuery.