In the style.css are css-classes, that are also used in React-Bootstrap.

.btn-group-xs > .btn, .btn-xs {
padding: .35rem .4rem .25rem .4rem;
font-size: .875rem;
line-height: .5;
border-radius: .2rem;
}

In my case using the component transformed all "xs"-buttons. It would be better to use styled-components inside the switch-component, which would only set the css there and eliminate side effects, Maybe I will try to improve the code later.