Bootstrap lets you pull down 1 CSS file and has a very opinionated design. It's not that easy to over-write its styles. The entire bootstrap.css minified is around 80kb, IIRC.

Pure is less than 15kb minified (< 5kb minified + gzipped). It's meant to be a starting point for your application's styles, taking care of all the hard stuff, like aligning forms, making buttons look consistent, having a common CSS base, etc. From here on, you can write your own application styles to make your app look pretty.

In the end, you end up using a lot less CSS to achieve a similar effect. in the end, they are different products. You can use Bootstrap and Pure together too (see the Extend page for an example).