Cropped file size is more than original

[GliderGuider]

I have a sample.jpg that is 731 KB. 

When I crop part of sample.jpg (Rectangle Select tool, then Image | Crop to selection) ,

the new saved cropped file is 1,969 KB.

 

Shouldn't the cropped file be smaller than the original?

Can anyone explain why the cropped file is MUCH larger than the original?

Is there any way I can maintain the original quality and end up with a smaller sized file after cropping?

 

[MJW]

I'd guess the the original image was more compressed. Before the image is saved as JPEG, PDN asks for the Quality setting. Perhaps setting it lower will help (though, of course, the image may look worse).

 

The main cause is likely a result of re-compressing a compressed image. When compressing the image, JPEG subdivides the image into 8x8 blocks. When you crop the image, the old block boundaries will likely fall within the interiors of the new blocks. The second JPEG compression ends up treating the JPEG compression noise from the first compression as data that must be encoded. The higher the original compression, the higher the JPEG compression noise, exacerbating the problem. Consider a simple example: a smooth diagonal color gradient. The gradient can be compressed into a small JPEG file. If you open the file in PDN, it will no longer be a smooth gradient; it will be blocky. If you crop it, the blocks will probably no longer align with the original compression block boundaries. When you re-compress the image, the compressor will be dealing with a more complex image than the original gradient.

Edited May 13, 2015 by MJW

[GliderGuider]

Interesting!  Makes sense.  Thank you for your insight.