Configurable Products
When using configurable or grouped products you can choose to pull information from the parent products. It's specified as 'Configurable products - non apparel' or 'Apparel' markup so activate the Apparel setting to enable the Apparel markup in your feed. Select in the multiple-select box below the fall-back data for the configurable / grouped products in the feed.

Using the multiple select field you can use the data where you want to use the parent data of the configurable product.
For example you have only 1 image on the configurable product but not on the simple products, in this way you can select the image_link as fallback on the simple products.

Keep in mind that you don't need to apply this to all the fields as then there aren't any variables anymore.
Most of the times the SKU, EAN, Size and Color needs to be loaded from the simple product itself.