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???
------Update-------
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.