Reducing file size of styled images
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