Reducing file size of styled images I always thought styled images are smaller than original ones in file size. So the main reason for styling was to reduce the file size. But today I noticed it isn't always the case. For example:

I downloaded an image abcd.jpg from facebook and then uploaded it to my Drupal website.

Originally the image was 405.3 kB which is 2048px X 1536px. 

After downloading the same image from my drupal website the image is 698 kB which is 1100px X 845px. 

Style Effect:(Scale width 1100)


And when I checked I found the same in most of the cases. Can anything be done about it without compromising image quality?
It's in the nature of image compression that compressing an already compressed image can result in larger filesize. The image-toolkit (be it GD or image magick) can not know on what level the image is already compressed and will try to make the best of it (following the quality setting that No Sssweat describes). So you may want to try to set this to 100% to prevent repeated compression. 

Further reading:


Efficient Image Resizing With ImageMagick 
In search of the most optimized image in Drupal 7