Use the variant, iconSize, or class attributes to customize the styling. For example, you can set class="slds-m-vertical_large" or other margin classes to add spacing around the icon
Or use your own class
<lightning:buttonMenu iconName="utility:settings"
iconSize="large"
variant="bare"
class ="slds-m-vertical_large"
alternativeText="Settings"
onselect="{! c.handleMenuSelect }">
Or use your own class