Having the same issue. Unfortunately, I don't think the feed that's sent to Google Merchant is capable of including variants like sizes. This is because they are not created in the shop. Each variant needs to have it's own page/link. Try this: Add the same product to your store but a different color or size.
When you hover over the product description link on the shopping cart page, notice the web address that comes up in the bottom left of your browser. Both items will have the same link. And that's the issue. If you do try this on a real site, you'll notice that each variant has a different web address