Possible approach to handling aspect ration differences when finding the best match for a requested image resolution in WordPress's image_get_intermediate_size function in media.php.

image_get_intermediate_size_possiblefix

/**
 * Retrieve the image's intermediate size (resized) path, width, and height.
 *
 * The $size parameter can be an array with the width and height respectively.
 * If the size matches the 'sizes' metadata array for width and height, then it
 * will be used. If there is no direct match, then the nearest image size larger
 * than the specified size will be used. If nothing is found, then the function
 * will break out and return false.
 *
 * The metadata 'sizes' is used for compatible sizes that can be used for the
 * parameter $size value.
 *
 * The url path will be given, when the $size parameter is a string.
 *
 * If you are passing an array for the $size, you should consider using
 * add_image_size() so that a cropped version is generated. It's much more
 * efficient than having to find the closest-sized image and then having the
 * browser scale down the image.
 *
 * @since 2.5.0
 * @see add_image_size()
 *
 * @param int $post_id Attachment ID for image.
 * @param array|string $size Optional, default is 'thumbnail'. Size of image, either array or string.
 * @return bool|array False on failure or array of file path, width, and height on success.
 */
function image_get_intermediate_size($post_id, $size='thumbnail') {
	if ( !is_array( $imagedata = wp_get_attachment_metadata( $post_id ) ) )
		return false;

	// get the best one for a specified set of dimensions
	if ( is_array($size) && !empty($imagedata['sizes']) ) {

		foreach ( $imagedata['sizes'] as $_size => $data ) {
			// already cropped to width or height; so use this size
			if ( ( $data['width'] == $size[0] && $data['height'] == $size[1] ) ) {
				$file = $data['file'];
				list($width, $height) = image_constrain_size_for_editor( $data['width'], $data['height'], $size );
				return compact( 'file', 'width', 'height' );
			}
			// calculate difference in aspect and resolution betweenn current image and requested resolution
			// values os zero are exact matches, higher numbers are worse
			$aspect_proximity = abs( ( $data['width'] / $data['height'] ) - ( $size[0] / $size[1] ) );
			$resolution_proximity = ( abs( $size[0] - $data['width'] ) + abs( $size[1] - $data['height'] ) ) / ( $size[0] + $size[1] );
			// add to lookup array
			$match_proximity = ( $aspect_proximity + $resolution_proximity ) * 100;
			$matches[ $match_proximity ] = $_size;
		}
		// sort the results, so the best match becomes the first element in the array
		ksort( $areas );
		// return the best match
		$data = $imagedata['sizes'][ $areas[0] ];
		$file = $data['file'];
		list($width, $height) = image_constrain_size_for_editor( $data['width'], $data['height'], $size );
		return compact( 'file', 'width', 'height' );
	}

	if ( is_array($size) || empty($size) || empty($imagedata['sizes'][$size]) )
		return false;

	$data = $imagedata['sizes'][$size];
	// include the full filesystem path of the intermediate file
	if ( empty($data['path']) && !empty($data['file']) ) {
		$file_url = wp_get_attachment_url($post_id);
		$data['path'] = path_join( dirname($imagedata['file']), $data['file'] );
		$data['url'] = path_join( dirname($file_url), $data['file'] );
	}
	return $data;
}