Does this need to be animated in jQuery? You can get your effect with CSS3 transitions (unless you're trying to support older browsers):
transition: all 0.3s ease;
-webkit-transition: all 0.3s ease;
-moz-transition: all 0.3s ease;
-o-transition: all 0.3s ease;
-ms-transition: all 0.3s ease;
Or font only:
transition: font 0.3s ease;
-webkit-transition: font 0.3s ease;
-moz-transition: font 0.3s ease;
-o-transition: font 0.3s ease;
-ms-transition: font 0.3s ease;
You can then use the .css() method to change font-size and the CSS transition will handle the animation.