The issue is that the padding is eating up the entire space inside the form control, which has a set fixed height.
You can make its height automatic, but your field will then be pretty big.
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" rel="stylesheet"/>
<div class="form-row">
<div class="col-md-6">
<div class="form-group">
<label class="small mb-1" for="selectType">Register as</label>
<select class="form-control py-4" id="selectType" style="height: auto;">
<option value="1">Student</option>
<option value="2">Mentor</option>
</select>
</div>
</div>
</div>