See https://core.trac.wordpress.org/ticket/59352

inline-image-size-fix.php

<?php
/**
 * Inline Image Size Fix Plugin for WordPress.
 *
 * @package   InlineImageSizeFix
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2023 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: Inline Image Size Fix
 * Description: When a large image is selected to be inserted in the block editor as an inline image, it is made artificially small. This can result in the image erroneously being given <code>fetchpriority=high</code> as well as <code>wp_img_tag_add_srcset_and_sizes_attr()</code> giving the image an incorrect <code>sizes</code> attribute, resulting in an oversized image being served. This fixes this issue. See ticket in WordPress Core Trac <a href="https://core.trac.wordpress.org/ticket/59352">#59352</a>.
 * Plugin URI: https://gist.github.com/westonruter/f37f747a57643462b7efe110dd61d91b
 * Version: 0.1.0
 * Author: Weston Ruter
 * Author URI: https://weston.ruter.net/
 * License: GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * Update URI: https://gist.github.com/westonruter/f37f747a57643462b7efe110dd61d91b
 */

namespace InlineImageSizeFix;
use WP_HTML_Tag_Processor;

/**
 * Fixes size of inline images.
 *
 * @param string $content String
 * @return string Fixed content.
 */
function fix_inline_image_size( string $content ): string {
	$p = new WP_HTML_Tag_Processor( $content );

	while ( $p->next_tag( array( 'tag_name' => 'IMG' ) ) ) {
		// Note this filter runs before wp_filter_content_tags so the width and height are not added yet.
		$src = $p->get_attribute( 'src' );
		$style = $p->get_attribute( 'style' );
		if (
			! $src ||
			$p->get_attribute( 'width' ) ||
			$p->get_attribute( 'height' ) ||
			! preg_match( '/^width:\s*(\d+)px;$/', $style, $style_matches ) ||
			! preg_match( '/^wp-image-(\d+)$/', $p->get_attribute( 'class' ), $class_matches )
		) {
			continue;
		}
		$style_width   = (int) $style_matches[1];
		$attachment_id = (int) $class_matches[1];
		if ( 0 === $attachment_id || 0 === $style_width ) {
			continue;
		}

		// Obtain the width and height as is done in wp_img_tag_add_width_and_height_attr().
		$image_meta = wp_get_attachment_metadata( $attachment_id );
		if ( ! $image_meta ) {
			continue;
		}
		$size_array = wp_image_src_get_dimensions( $src, $image_meta, $attachment_id );
		if ( ! $size_array ) {
			continue;
		}

		$new_height = ( $style_width / $size_array[0] ) * $size_array[1];
		$new_width = $style_width;
		$p->set_attribute( 'width', $new_width );
		$p->set_attribute( 'height', ceil( $new_height ) );
		$style .= sprintf( 'height: %fpx;', $new_height ); // Since the height may be fractional now.
		$p->set_attribute( 'style', $style );
	}

	return $p->get_updated_html();
}

add_filter(
	'the_content',
	__NAMESPACE__ . '\fix_inline_image_size',
	has_filter( 'the_content', 'wp_filter_content_tags' ) - 1
);