WordPress (5.3+) likes to scale down all uploaded images added to the media library.
To disable this feature add this code to your theme functions.php file
add_filter( 'big_image_size_threshold', '__return_false' );
To increase the threshold instead, add the following to your theme functions.php file:
function pands_big_image_size_threshold( $threshold ) {
return 4000; // new size threshold
}
add_filter('big_image_size_threshold', 'pands_big_image_size_threshold', 100, 1);