I rig the XXS completely, Shift+D to duplicate, and 'M' to move to the layer (I use the top row of five layers, one for each of the standard sizing bodies and the five layers directly below them for each size of the garment) I'm using for my larges. (I never unparent them from the armature). Then I pretty much follow the Siddean Munro tutorial to the letter to make the shape key sizes. (She does make shape keys, she adds the first shape key, a 'BASIS,' then adds a second shape key named large and sets the value to ONE before sizing, then drags the slider down to create the next size down, and creates a new shape key from that mix - I reccomend watching the video on fullscreen if you didn't already). I duplicate the shape keyed garment to my Medium, Small, and X-Small layers (I don't bother with the XXSmall shape key since by this point, I already have a fully weighted XXS), and delete the shape keys (the shape key you want to keep should be the last one you delete). Then, once I have my sizes, I upload each one to beta and wear it with my alpha layer and corresponding shape and tweak the weighting on each size as needed. Once everything is weighted correctly, I line what needs lined and export/upload. Hope that helped