Seller has forced the bust measurements to be made bigger.
Open a Not As Described dispute.
You don't have to prove anything.
The seller is required to send you return shipping.
If they are in the USA they can still use pirateship or another provider to make a label, or ask eBay for help, or tell you to keep the disputed sweater.
They will be required to refund you.
EBay is a mediator not a judge.
Just open the Claim- Not as Described.
EBay will back you.