But in addition, there also seems to be much more compact HTML. I don't know if that is actually the case.
Compare:
<button type="button" class="btn btn-primary"> Primary </button>
<button class="ui button"> Follow </button>
or
Semantic UI:
<div class="ui breadcrumb"> <a class="section">Home</a> <div class="divider"> / </div> <a class="section">Store</a> <div class="divider"> / </div> <div class="active section">T-Shirt</div> </div>
Twitter Bootstrap:
<ol class="breadcrumb"> <li class="breadcrumb-item active">Home</li> </ol> <ol class="breadcrumb"> <li class="breadcrumb-item"><a href="#">Home</a></li> <li class="breadcrumb-item active">Library</li> </ol> <ol class="breadcrumb"> <li class="breadcrumb-item"><a href="#">Home</a></li> <li class="breadcrumb-item"><a href="#">Library</a></li> <li class="breadcrumb-item active">Data</li> </ol>