You're doing something more than simply changing the dpi setting if the file size is increasing significantly. Just changing the dpi setting in the image's metadata should not affect file size if the number of pixels in the image is the same.

(Just for the record, the value listed in the EXIF standard as 'dpi', or "dots per inch," is really ppi, or "pixels per inch." For more please see How do I generate high quality prints with an ink jet printer?)

My guess would be that you are re-rendering a fairly compressed jpeg and then saving it at a much higher quality level that uses much lower compression, but that wouldn't usually result in files sizes in the 40MB range unless your images are about 100MP! So it seems there is something else in play. Perhaps you are using another image format, such as PNG or TIFF, that often requires larger file sizes for the same image resolution?

You have the "resampling" option checked in Photoshop's 'Resize Image' dialog. If that option is selected then Photoshop will increase the number of pixels in the image file to match the increase in the "DPI" setting. If your images were set at "72 DPI" and you increase the "DPI" setting to "300 DPI" then the number of pixels along each edge of the image will be multiplied by a factor of 4.167X. That means the total number of pixels (length x width) will increase by a factor of 17.36X! Your uncompressed image that was originally about 0.7MP was exported as a 12MP image!

To keep the same number of pixels but change the "DPI" setting, uncheck the "resampling" option in the 'Resize Image' settings before converting the images.

This will result in a smaller display size of the image, though. At 300 pixels per inch, your 720 x 960 pixel image will only take up 2.4 x 3.2 inches on a printed page of a book.

If you want the image to be 10 x 13.333 inches then you need to supply the publisher with 3000 x 4000 pixel image files with a "DPI" value of "300."

If you still have access to the scanned documents, my advice would be to rescan them at a high enough resolution make them 3000 x 4000 pixels without needing to resize them and exporting the image in a format that will allow your 12MP images to be smaller than 30+ Megabytes! Particularly if they are all black and white with no color information, there is no need to save them in a full color uncompressed image format as appears to have been the case.

In a publishing environment, the designated "dpi" has real meaning because desktop publishing applications work in real world dimensions such as 'inches' and 'centimeters'.

This is something most of us are no longer used to when working with digital images on a screen or sending directly to a printer. This is because most applications can and do automatically rescale an image to fit a computer screen or to fit a specific size printing paper.

But in a physical publishing environment, many of the applications/programs used to set page layouts actually read and apply the photo's ppi setting (designated as "dpi" in the image metadata) to set the size of the image on the page. Many word processing applications do the same thing if you insert an image into a document.

An image that is 720 pixels wide will appear to be 10 inches wide if the "dpi" is set to '72', but will only appear to be about 2.4 inches wide if the "dpi" is set to '300'. For more, please see: Is there any practical difference between saving at 300ppi or 72ppi?

In such a case, you need to:

• Determine the actual physical dimensions you want the image to have on the printed page.
• Resize your image so that the number of pixels along each edge is (300 x the number of inches desired) and confirm that the image metadata includes '300' in the "DPI" field.