views/macros/button_group_classes.html.twig
{% macro button_group_classes(size, type, extra_classes) %}
btn-group
{% if size == 'xsmall' %}btn-group-xs{% endif %}
{% if size == 'small' %}btn-group-sm{% endif %}
{% if size == 'medium' %}{# No need for an extra class #}{% endif %}
{% if size == 'large' %}btn-group-lg{% endif %}
{% if size == 'xlarge' %}btn-group-lg{% endif %}
{{ type }}
{{ extra_classes }}
{% endmacro %}