Product size ordering using product weight

[onnj]

Hello everyone i'm using Joomla! 2.5 and virtuemart2 and I have products with different sizes so what I did is I used the custom field and name it as size all settings are correct now my problem is if I will use the custom field and filter the child products by Product SKU the Large size will come first then the Medium size because in the alphabet L first before M. So what is did for the solution is I set weight for small, medium, and large product to .5kg, 1kg, 1.5kg and set the custom field to filter the child products to weight but still Large comes first before medium. I tried using this to other products and the orderings are fine medium first then large but to some products its not working.

Can you please help me guys how will I solve this problem because I think its a bug because to some products it works but to some it doesn't. Can you please give me other option how will I set the order of the size of my products from small to large.

Small (Parent)
Medium and Large (Child Product)