Hi @katandmouse ,?
Because themes?have different needs and image providers have a limited number of sizes they provide, we use the full version for the initial download. Its advised to use the original in case over time you want to add more sizes. However, stockpack will abide any image compression plugin you use, as well as the max size for images via the big image threshold:
https://www.itsupportguides.com/knowledge-base/wordpress/using-wordpress-big_image_size_threshold-php-filter/
Let me know if you have any other questions.