If you sell a product on Bol.com that is available in different sizes, your sizes might turn up separately on Bol.com. If you want your sizes to be grouped together and displayed as one product available in different sizes on Bol.com, you will need to create a "productfamilie" for these variant products.
You can follow the manual to setup "productfamilies" that Bol.com has provided on their website (website and manual are only available in Dutch).
Bol.com also supports delivering "product families" 'in bulk', according to their documentation. The "Family Key" mentioned in their documentation can be found in Channable in the Attributes step. The attribute for this is called "Productvariant ID".