That's a great question!
Shopify does indeed compress images when they display on your website, they provide details on that on the Uploading images help page here: https://help.shopify.com/en/manual/online-store/legacy/using-themes/change-the-layout/images
We also provide additional recommendations for using third-party apps and tools on our blog here: https://speedboostr.com/how-to-compress-images-on-shopify/
One benefit to using an app like Crush Pics is that it also improves image SEO, you can read more about that here: https://speedboostr.com/shopify-image-seo/
Hope this helps!
Thank you for your reply
For testing purpose I have just compressed an image, my new file size is 21kb.
Same is the size in shopify
My question is what is the minimum image size when shopify default compression behavior stops?
I have already done extensive research but doesn't find solution.