Saving PSD file (670MB) heavily reduces file size of PSD (256MB)

[Houndrock]

A PSD file I created (with Keyshot) has a file size of 670MB. However, when I save it again under a new name, the file size gets reduced to 256MB, without any edits done. 

First, I thought it had to do with the file handling preferences 'Maximize compression of PSD and PSB Files', or 'Disable compression of PSD or PSB files', but after testing it out with the options checked/unchecked, it still saved the file to around 256MB in any case.

Does anybody have an idea why Photoshop is reducing my file size when saving the file into a renamed Photoshop file?

Thanks for the help!

-Dan

[Stephen_A_Marsh]

What is the width and height of the image in pixels (resolution is irrelevant).

What is the bit depth of the image?

What is the colour mode?

How many layers?

How many channels?

[Houndrock]

Hi Stephen,

Thanks for your reply!

Width and height of image: {44.85 x 21.6}cm at 300 Pixels/Inch

(Strangely enough it was shown as {186.97 x 90.04}cm at 71.96 Pixels/Inch).

Bit depths: 8 Bits/channel

Colour mode: RGB

Layers: 12 hidden, and 1 active. 

Channels: The usual: RGB, Red, Green, Blue

[Stephen_A_Marsh]

This should deliver a 5297 x 2551 px file at 38.7 Mb for a single layer when opened in Photoshop. On-disk this would be ~77.4 Mb. I believe that Photoshop uses lossless LZ compression for PSD/PSB images. There is a preference setting to turn off compression when saving as PSD/PSB.

13 identical layers of full canvas content created an  ~543.3 Mb sized file on disk.

Have you compared the metadata of both files to see if this could account for the smaller file size?

[Houndrock]

Hi Stephen,

Thanks for your reply! Sorry for the late reply on my side. My notifications keep not coming through.

I tried the different options, with the results shown below for each PSD file.
It seems that 'Disable compression of PSD or PSB files' didn't make a difference, and that 'Maximize compression of PSD and PSB Files' increases the file size by ~47MB when checked.

However, that does not explain the reduction of ~450MB whenever the original is saved. Two other options that are always checked in the file handling preferences are: 'Prefer Adobe Camera Raw for Supported Raw Files, and 'Ask Before Saving Layered TIFF Files'. Unchecking 'Prefer Adobe Camera Raw for Supported Raw Files' also didn't make any difference in the file size.

I'm stumped.

Below the results:

>Original
File size: 670.75 MB
Dimensions: 5297x2551
Dimensions in cm: 186.9 cm x 90.0 cm
Resolution: 72ppi
Bit Depth: 8
Color Mode: RGB
Color Profile: Untagged

>Disable compression: unchecked; maximum compatibility: never
File size: 219.68 MB
Dimensions: " "
Dimensions in cm: " "
Resolution: " "
Bit Depth: " "
Color Mode: " "
Color Profile: " "

>Disable compression: unchecked; maximum compatibility: always
File size: 256.66 MB
Dimensions: " "
Dimensions in cm: " "
Resolution: " "
Bit Depth: " "
Color Mode: " "
Color Profile: " "

>Disable compression: checked; maximum compatibility: never
File size: 219.68 MB
Dimensions: " "
Dimensions in cm: " "
Resolution: " "
Bit Depth: " "
Color Mode: " "
Color Profile: " "

>Disable compression: checked; maximum compatibility: always
File size: 256.66 MB
Dimensions: " "
Dimensions in cm: " "
Resolution: " "
Bit Depth: " "
Color Mode: " "
Color Profile: " "

[Houndrock]

Bump! Anyone who could have an idea of what it could be?