What is the "optimal" file size of JPEG images with respect to their dimensions?

I plan to write a script that will scan 100,000+ JPEG images and re-compress them if they are "too big" in terms of file size. Scripting is the easy part, but I am not sure how to categorize an image as being "too big".

For example there is a 2400x600px image with a file size of 1.81MB. Photoshop's save for web command creates a 540KB file at 60 quality and same dimensions. This is about 29% of original size.

Now I am thinking about using these numbers as a guideline. Something like 540KB / (2,400 * 600 / 1,000,000) = 375KB per megapixel. Any image larger than this is considered big. Is this the correct approach or is there a better one?

Edit 1: the images need to be optimized for display on websites.

Edit 2: I can determine the desired output quality by experimenting, I need to know if the images are big in terms of file size w.r.t dimensions and need to be saved in lower quality.

Answer

On average, JPEG's sweet spot is around one bit per pixel.

This will of course vary depending on image content, because certain types of graphics (e.g. flat areas and smooth gradients) compress better than others (noise, text), so it's not a robust method to apply blindly to every image.

You also have a problem of not having an uncompressed reference image to compare with, so you don't really know for sure what's the current quality of the images you have, and how much more you can lower the quality to be still acceptable. The quality can be guessed to a certain extent from quantization tables in JPEGs, but it's not a reliable method either (specifically, ImageMagick's quality judgement is very incorrect for JPEGs with custom, optimized quantization tables).

Having said that, there is a reasonable practical approach:

1. Pick a maximum JPEG quality setting you're happy with (somewhere in 70 to 85 range).


2. Recompress images to that quality level.


3. If the recompressed image is smaller by more than ~10%, then keep the recompressed image.

It's important not to pick merely the smaller file size, and require a significant drop in file size instead. That's because recompression of JPEG tends to always drop the file size slighly due to loss of detail caused by lossy nature of JPEG and conversion to 8-bit RGB, so small drops in file size can have disproportionally large drop in quality that isn't worth it.