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There could be several reasons for that, but in a nutshell it is probably because the default save parameters are not the ones used to create the original image.
It could notably be:
- The original image was created with a higher quality. The default quality of Pillow is 75 (it can go from 1 to 95 (best)). A lower quality can sometimes reduce the size.
- The original image had EXIF data (from a camera for example) or an ICC profile. By default, Pillow does not save any EXIF data or ICC profile.
See the complete list of options of the save method for JPEG file, and their default value, and you should find out which one is different in your case.
Source:stackexchange.com