This issue is caused by extra style added to listings as suggested by the ebay mobile friendly checker. eBay was requesting that a max-width style be applied to all images in a listings, and we were complying with that request, but have since removed that specific styling as it could produce issues like what you've noted here. Newly created listings will not have this style tag applied, so going forward your logo should have the correct sizing. If you use the eBay mobile friendly checker, it may say something like "Your listing isn't quite mobile friendly. Let fix it!" Just don't click the "Fix it for me" button, as it will add that style tag back into your listing and in your case, you don't want that style applied to your listing.
Sorry for the inconvenience.