Recently, I have started to look into image optimisation. Specifically, optimising images for the web.

Using Photoshop, I have come to notice that when you open a new image, the file size could appear around the 45kb whilst in Photoshop, it can appear at over 2mb. That's a big difference.

Furthermore, I can then save the image for the web, where I can reduce the file size by over 75% in Photoshop but when I then look at the file size on my Desktop, the image either has negligible difference or can sometimes appear larger than the original, despite Photoshop indicating that the image has been reduced.

How can all this be? Am I missing something here?

  • 1
    This obviously doesn't answer the question, but maybe you'll benefit from this info too. Third party apps can shave off some extra kilos... I use Image Optim on Mac. if you're not on Mac, check out the big blue "See alternative versions" button.
    – Joonas
    Sep 10, 2018 at 6:08
  • What file format is the image you are opening?
    – Billy Kerr
    Sep 10, 2018 at 10:23
  • @Joonas: Thanks for the insight and suggested resource.
    – Craig
    Sep 10, 2018 at 22:49
  • @BillyKerr: The images are made up of a variety of .jpeg and .png file formats. The .png file formats and predominantly for images with a transparent background since .jpeg does not support this.
    – Craig
    Sep 10, 2018 at 22:50
  • So, jpeg and png are compressed image formats. Jpeg uses lossy compression, and png uses lossless compression. When you open them in Photoshop it opens the compressed images at full size, by uncompressing them. This is normal, nothing is wrong.
    – Billy Kerr
    Sep 11, 2018 at 8:05

1 Answer 1


Some formats are compressed formats - Tiff, JPG, PNG, PSD, etc., all use compression. Therefore the image size reported for an open image in an application is often the uncompressed size, whereas on the desktop, as a file, it's the compressed size.

In addition, hard disk drives have minimum block sizes. So, for example the hard drive itself may have a minimum block size of 4 bytes. So even a 2 byte image will be reported as 4 bytes (the minimum) when examining the file in the OS.

You can see something like:

  • Size: 220,233 bytes (221 KB on disk)

Where the actual size is reported and the "on disk" size would indicate the use of minimum blocks for the size. Sizes are always rounded up to the nearest block size on a hard drive.

On the Macintosh, the Finder will always show the "on disk" size. You can only ever see the actual size by using Get Info on an image file. (I would assume Windows Explorer is similar, but, well, I don't do Windows).

  • Thanks for this answer. With particular reference to WordPress websites, would I be right to assume that this Content Management System also compresses files by default? I am thinking this, since when I upload the images, they are showing the file sizes around 60kb, for images which are over 2mb in applications such as Photoshop?
    – Craig
    Sep 10, 2018 at 22:48
  • 1
    @Craig customarily any image upload on a site will process the image through something like Imagemagick which can be configured to compress images as well. Essentially it has a "jpg quality setting" similar to Photoshop's, which will alter the image quality in relation to file size. So, uploaded images are typically an unknown because there's no direct way to know what settings the upload script may be configured to use - unless you own the website and change the settings.
    – Scott
    Sep 10, 2018 at 23:21

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