Semantic UI author here. The thing I think most important in considering SUI over Bootstrap, is its capacity to be themed/extended. See our examples of GitHub

Semantic UI author here. The thing I think most important in considering SUI over Bootstrap, is its capacity to be themed/extended. See our examples of GitHub layout clones in SUI (be sure to hit paint can icon in top right to swap in other themes) Modern GitHub http://semantic-org.github.io/example-github/index.html http://semantic-org.github.io/example-github/index.html Classic GitHub http://semantic-org.github.io/example-github/index-classic.html http://semantic-org.github.io/example-github/index-classic.h... SUI is designed with 3,700+ variables under the hood, and a six level deep theming system (site default theme -> site packaged theme -> site user theme -> component default theme -> component packaged theme -> component user theme). It becomes very possible to build large codebases around SUI that can be extended over the course of a startups growth that can build all of the company-specific changes purely in theming variables, custom components and css "override" files. After setting up your core UI in SUI you can simply fork our doc style guide to use as branded style guide for your company. Deploying a new "on brand" microsite can be as simple as copying over a theme folder, tweaking things like @primaryColor @fontSize, @fontFamily, and changing some button shading to create a new variant. With regards to the build size, most of why we are larger is that SUI includes 50+ components. With our gulp pipeline you can narrow it down to only the components you want. There's even a PR that I'm looking at which, when merged, will let you specify only the specific parts of a single component you want to include in your build, narrowing down file size even more.