Large pic field images have been scaled down automatically to 678x300px

[medbasa]

There's a problem with the large image fields (in our case, 1800x600px) after the newest WP update.

Screen 1. The image uploaded has/had the right size of 1800x600px. It looks pixelated on the front-end now.
Screen 2. Upon opening one can see the image was somehow resized to the small image field size (678x300px)
3. If try to change the image to a big one from WP Gallery - downsized again.

Please fix, thanks