Image Attachment is Scaled, but I'd rather it not be

Hi! I am using the following code to display image attachments:

<?php echo wp_get_attachment_image(0, 'large', false); ?>

I’ve set “large” size to be 900 pixels wide, which fits great in the one-column theme of my site. For some reason, it’s scaling the image to 675 pixels wide.

How can I turn off scaling?

Thank you!

(For reference, here’s one of my sample attachment pages: https://craftcore.ca/2012-05-01-sample-post-with-tags/apeture-guide-to-user-experience/ )