change_size($('#fruits'), 'bigger');
// ^ add missing (
You can optimize your
change_size
function like this.
function change_size(element, sizeName) {
var current_size = parseInt(element.css('font-size')),
new_size = (sizeName == 'smaller') ? current_size - 4 : current_size + 4;
element.css('font-size', new_size + 'px');
}