I have been looking at the file size when selecting different resolution in Flamingo nXt,
Display ~ 72 dpi
Printer, draft ~ 100 dpi
Printer, normal ~ 150 dpi
Printer, high quality ~ 300 dpi
Usually I find out what size people want to print the file and scale up the image (A4 = 8.3 x 11.7 x 300 dpi = 2490 x 3510 image size in pixels).
Just because someone wanted a different print size I had to look at the file in Photoshop, noticed that the dpi was wrong. So I wondered if the file size would increase as I can the dpi, it does, all three dpi (100, 150, 300) had the same file size like my test ages ago [Resolution and file size][1]
The logic would be the higher the dpi, the bigger the file size, that is not the case. Scott Davidson said it was a simple a bug, looks like it hasn’t been fixed.
If you specify the pixel dimensions, changing dpi doesn’t change the file size. File size will only (mainly) depend on the number of pixels. The dpi stored in the file is just a hint for the printer or image processor as to the intended the physical dimensions of the image.
If you render your image to A4 at 300dpi but print that on A2, the resolution of the physical print will be 150dpi. Your file might have stored 72dpi as a reference, that doesn’t matter. Some file formats don’t even have the ability to store image data such as dpi…
Basically the number of pixel defines the file size.
Other than that, compression, content and quality may influence the size of certain file formats.
Make sure you have the correct input.
If you specify pixel dimensions, dpi settings will change the (intended) physical dimensions of the image. So file size doesn’t change.
Of you specify a physical dimension, dpi settings are used to calculate the number of pixels necessary to fulfill that requirement. Since higher dpi result in more pixels for the given physical dimension, the file size will increase.
If you specified physical dimensions and dpi but the pixels don’t change, make sure, you don’t have any option activated that might override the pixel calculation. If you are sure the is no custom pixel dimension in any way, then the dpi-to-pixel calculation is bugged and you will have to do that simple calculation on your own.