I think the default pre paid return label eBay provides is an upto 2kg small parcel RM 48hr tracked.
If the parcel is bigger the seller really should provide you with the correct wighted and sized label.
You need to contact eBay customer services and tell them you were given an under weight label and the seller doesn't want to help you.
Here's a link to customer services that usually bypasses the automated assistant during opening hours.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
The seller being charged already for the wrong sized label is their problem not yours, but they should be able to get a refund for it as long as it hasn't been scanned.