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

If I use DESC instead ASC I have this sorting:

25g
10g
100g