If you dig out the code with firebug, you can find out that the ul children are like ul> li> a> span> span . And in the last span tag you will have css as follows.
.localnav .ui-btn-inner { font-size: 80%; }
If you change the font size to less than% or in px, you can get the required size. I think the following will work
$('ul li li a span span').css('font-size',60%');
I did not make any example. Please ignore this if it does not work for you.
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