Bootstrap CSS is created and maintained by Twitter, which is famous for consuming megabytes of bandwidth, to display a couple of 160 characters long messages in your browser. Check out the difference below, first Micro.
The Bootstrap CSS (notice Google seems to be down at the moment, at least from my home, so there are several crucial files served over their CDN which aren’t loaded).
However, unless my memory plays tricks with me, Bootstrap CSS is at least 30 times as large in total as Micro.
The problem with Bootstrap CSS (and why you should never use it on new projects), is that it is built upon an archaic and ancient technology. And upgrading it to use the new “flexbox model”, is impossible, due to “legacy problems”. Making Bootstrap become a modern CSS framework, would basically make every single webpage out there consuming it break. So Bootstrap is cornered, and cannot upgrade their code to become modern, without creating breaking changes, making it impossible to update from older versions of Bootstrap to their new version.
There are hundreds of interesting CSS frameworks out there, besides Bootstrap. Milligram being one for example. The one I am using is called Micro, and is Open Source and integrated into the main download of Phosphorus Five. And yes, I created it. If you want to use it in commercial projects though, you’ll need a valid proprietary license for Phosphorus Five.