Reduce thumbnails weight

[Rostom]

We are uploading all our product pics all in the same width and the same file weight range

But I realized that Shopify automatically-created thumbnails vary a lot in file weight sizes, some of them reach up to 90 KB, and some at 50 and 40 KB, and some at 25 KB for no apparent reason. How can I enforce or control or reduce the file weight of my product thumbnails? is there's a quality settings I should change somewhere, or max file size I should set?

Thank you.