@rifton007 posted a quick fix in the GitHub issue on this subject. Add this in your css file:
.btn-group-xs > .btn, .btn-xs {
padding: .25rem .4rem;
font-size: .875rem;
line-height: .5;
border-radius: .2rem;
}
Demo:
<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" rel="stylesheet"/>
<style>
.btn-group-xs > .btn, .btn-xs {
padding: .25rem .4rem;
font-size: .875rem;
line-height: .5;
border-radius: .2rem;
}
</style>
<button class="btn btn-primary btn-xs">♡</button>
<button class="btn btn-primary btn-sm">♡</button>
<button class="btn btn-primary">♡</button>
<button class="btn btn-primary btn-lg">♡</button>
source
Edit:
later on the issue, @deadmann replied that he dig up a little in old BS3 documentation and extract the following code:
<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" rel="stylesheet"/>
<style>
.btn-group-xs > .btn, .btn-xs {
padding: 1px 5px;
font-size: 12px;
line-height: 1.5;
border-radius: 3px;
}
</style>
<button class="btn btn-primary btn-xs">♡</button>
<button class="btn btn-primary btn-sm">♡</button>
<button class="btn btn-primary">♡</button>
<button class="btn btn-primary btn-lg">♡</button>