Is it possible to get feature image url (full-size) from an image url of smaller size?

The URL for rezied images in WordPress is constructed in a way that’s impossible to direct reverse. Therefore, it’s impossible to get the attachment associated with a URL of a resized image with 100% certainty, unless you loop through all attachments and check whether their URL matches the given URL. However, this approach is very slow and resource-intensive.

In any normal WordPress setup, you can rest assured that, given a URL of a resized image, you can retrieve the full image URL by removing the dimensions from the URL. This is, however strange it may seem, best practice.