The reply offered by _sashok is right. If you use in the way you ask about you are going to have many problems.
In the website you ask about really they are using items, they aren't using weight, litres or lenght, that info is supplied in the product item.
If you have a product with 125 ml and another with 200 ml you should use attributes so the customer can select in the same item the two options and the quantity he wish one, five or one hundred items
Try change the structure only going to give you headache. Seo, google shopping, amazon, any comparising prices website
If you sell in bulk in KG you can stablish 100 gr so customers have to buy 10x100gr to buy a kg you too can establish minimum unit to buy so they have to buy a minimum of 10x100gr.
You too can play with the description to offer a long description to help cutomer.