Why is image size becoming larger after cropping it in PS?

Question

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.

Answer 1

The "original size" you are seeing isn't a JPEG, it'll be either a PSD or a TIFF. Photoshop isn't looking at the original file, it's looking at the image that's currently open in the application, and that's an uncompressed image.

The reason why you're seeing a larger file size when you try to save a new JPEG version is that the JPEG you're working with is badly compressed with a lot of artifacts. Photoshop (or, rather, the JPEG compression algorithm) sees those artifacts as image data and will try to preserve them. Basically, the crappy picture seems to be more complicated than the "original original" clearer picture was, so as counterintuitive as it may seem, you'd wind up with a smaller file in the end if you started out with a much larger one.

There is a Photoshop tool (and there are better plugins) that can "de-JPEG" the image to a degree, but the image is small and the artifacting here is pretty severe, so it's highly doubtful that you'd see much improvement - cleaning up the artifacts automatically would probably destroy the image. If you put some work into it, you can probably kill some of the "ringing" and blockiness manually, but you really have to watch that you don't turn it into a painting as you're doing so. The smaller an image is, the harder it is to keep it looking like a photograph.

Answer 2

the pixel dimensions and the file size change but the image isn't resampled. ... That's because the number of pixels per inch can vary based on the number of pixels in the crop selection area.