When returning the URL, ACF will only return the URL of the original image.
You need to either return the image array from the image field so that you have more to work with, the documentation includes information on doing this https://www.advancedcustomfields.com/resources/image/
or you need to return the image ID can get the image size you want using wp_get_attachment_image_src() https://developer.wordpress.org/reference/functions/wp_get_attachment_image_src/
