I have configurable product create with attribute size, I try to list this attribute size in the product description, but addAttributeToSort() not working. My code is:

    if($_product->getTypeId() == "configurable"):
        $conf = Mage::getModel('catalog/product_type_configurable')->setProduct($_product);
    
        $simple_collection = $conf->getUsedProductCollection()->addAttributeToSelect('*')->addAttributeToSort('size','ASC')->addFilterByRequiredOptions();

right now I have something like this:

    100g
    10g
    25g