I am using photoshop to crop Youtube preview images like this one, which is a .jpg file, the url being "http://img.youtube.com/vi/g-LvhmaHEWA/0.jpg"
Originally it is 480*360 and about 25k on disk, what I do is change the canva size to 480*270 so that the black bar will be cropped out. But after saving the file as .jpg file too, its size turns to 100+k. I changed the image quality to the lowest and still it is about 40k.
Orignal downloaded image (25k)
Trying to save with "save for web Legacy" (why the original is 380k???)
I guess the key problem is the original size of the picture shows as 380K, why???
This answer of this question states that for photographs it is more in line with intuition. But in my case, the image becomes about 2 times the original size, it is total counter-intuition. Moreover, I've tried open a 21K youtube preview image in PS, and directly save it without any editting, with image quality being 0, the file becomes 30+k for all "format options". So I think the rational behind image becoming larger is different from what that question answers.