.form-control-sm and .custom-select-sm give different font size/padding

Created by: alecpl

I expect these two to give the same font-size and left-padding:

<select class="form-control form-control-sm">
  <option>Small select</option>
</select>
<select class="custom-select custom-select-sm">
  <option>Small select</option>
</select>

https://jsfiddle.net/6oj656wb/