Let's stretch the cards by setting the flexible attribute of the component to auto:
<aura:application extends="force:slds"> <Lightning:layout verticalAlign="start"> <Lightning:layoutItem flexibility="auto" padding="around-small"> <Lightning:card title="1"> <p class="slds-p-horizontal_small"> Card 1 </p> </Lightning:card> </Lightning:layoutItem> <Lightning:layoutItem flexibility="auto" padding="around-small"> <Lightning:card title="2"> <p class="slds-p-horizontal_small"> Card 2 </p> </Lightning:card> </Lightning:layoutItem> <Lightning:layoutItem flexibility="auto" padding="around-small"> <Lightning:card title="3"> <p class="slds-p-horizontal_small"> Card 3 </p> </Lightning:card> </Lightning:layoutItem> ...