Today’s Question: Why is it that a JPEG on my desktop says its 947KB and when I open it in Photoshop it says it is 14.4MB?
Tim’s Quick Answer: In this case Photoshop isn’t reporting the actual file size on your hard drive, but rather the total amount of data based on pixel information. In other words, Photoshop is not taking into account the compression that will be applied when you save an image as a JPEG file.
More Detail: The file sizes that can be displayed on the status bar below the image window in Photoshop don’t actually report file size, but instead report what is referred to as the document size. There are actually two document sizes displayed, which can provide you with an idea of what sort of file size to expect for an image, but only for certain circumstances.
If you display the Document Sizes option on the status bar below the image, the file size indication on the left represents a flattened version of the image, and the file size indication on the right represents a layered version of the image.
In other words, the first number can be thought of as the file size you could expect if you saved the image as a TIFF file with no compression applied, but with all layers flattened. The number on the right is the estimated file size if you save the file as a TIFF image without compression and with all layers intact.
When you save an image as a JPEG image, it will always be a flattened version of your document in Photoshop, because JPEG images can’t contain layers. In addition, JPEG images always have compression applied to them. That compression helps to reduce file size, though it also has a negative impact on overall image quality. That is why a JPEG is good for sharing in certain contexts, but not ideal as a “master” image file format.
In addition, you can adjust the “Quality” setting for a JPEG image, which impacts not only the overall image quality but also the file size. But again, the Document Sizes information display that is available for the status bar for your photos relates more to the amount of information contained in the image you’re working with, which doesn’t directly translate into the actual size of the file that will be saved on your hard drive.