Hi @bonzai89,
The 503 Service Unavailable error message displays when Web server (running the Web site) is currently unable to handle the HTTP request due to a temporary overloading or maintenance of the server.
You will find more information about this error message on the following page.
https://www.checkupdown.com/status/E503.html
Can you please try enabling WP_DEBUG mode? To do this, just go to wp-config.php include this:
define('WP_DEBUG',true);
You might already have a line for WP_DEBUG, so just make sure it’s set to true.
For information on this, checkout this wpmu.org article.
Let me know what errors if any you are getting bulk smushing images on clicking on the bulk button.
This can also be due to theme/plugin conflict.
You can just try on your development site using default WordPress theme like Twenty Thirteen and deactivating all other plugins as displayed in the following flowchart to know which theme/plugin is conflicting if any.
https://premium.wpmudev.org/wp-content/uploads/2013/12/Support-Process-Support-Process.png
Regards,
WPMU DEV