Increasing font size causes submenu to overlap with top level menu

I've increased the size of the font for a menu by doing:

.sf-menu li a {
font-size:1.2em;
}

but that's causing the submenu below it to cover the menu item directly above it.

Not getting much luck with whatever css I'm throwing at it. I actually wanted the font to be bigger but that makes the overlapping worse so that the top level menu item is almost entirely covered when the submenu pops out.