One effective method to decrease the file size of a Photoshop document is to conceal all layers. This technique was introduced to me by Mikolaj Podlasek, a designer and author at Design Instruct. While sharing a Photoshop file via email, he informed me that all layers were hidden to minimize the PSD’s file size.
There are various ways to hide all layers in a PSD file.
A straightforward approach is to select all layers within the Layers panel and then choose Layer > Hide All Layers from Photoshop’s main menu. When all layers are hidden, the canvas will appear blank, and there will be no “eye” icons visible to the left of the layer names. 
To confirm that all layers are hidden, simply check the canvas and layer icons. After hiding the layers, resave the PSD by selecting File > Save (Ctrl + S).
Upon saving, the new file size should be reduced, indicating that the Photoshop file is now smaller. 
To verify the effectiveness of this technique, I tested it on three PSD files from our freebies archive:
Free PSD: Toucan UI Kit
Free Stationery Mock-Up Template (PSD)
Free PSD Template: Presentation Folder Mockup
In each instance, the Photoshop file size was reduced when all layers were hidden, with savings ranging from 14% to 33%.
Original file size
File size with layers hidden
% file reduction
Toucan UI Kit
6.16 MB
5.18 MB
15.9%
Stationery Mock-Up Template
69.7 MB
46.8 MB
32.8%
Presentation Folder Mockup
48.0 MB
41.1 MB
14.4%
For even smaller file sizes, consider compressing your PSD using a file archiver like 7-Zip. I utilized this free and open-source tool to create zip archives of the PSD files.
The following table summarizes the file sizes after compressing the PSDs:
Original file size
File size with layers hidden + zipped
Total File reduction
Toucan UI Kit
6.16 MB
1.35 MB
78.1%
Stationery Mock-Up Template
69.7 MB
18.8 MB
73.0%
Presentation Folder Mockup
48.0 MB
10.6 MB
77.9%
By applying these two techniques, PSD file sizes were reduced by up to 78%.
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