Jane,
You would have to increase the img height and width and remove or make the margin-top smaller.
So something like add this new css rule to the end of the common.css file in the launchpad theme.
.claro .jimu-anchorbar-iconitem .icon-item .icon {
width: 30px !important;
height: 30px !important;
margin-top: 5px !important;
}