If you go to the settings on the Preferences dialog for File Handling, one of the options is Maximize PSD and PSB File Compatibility. The typical advice is to set this, if you want your .PSD files to be backwards compatible with earlier versions of Photoshop. There's other reasons for setting this option to Always or Ask.

This feature will increase your file size. What it does is save a flattened version of the image inside the file. That's how Photoshop ensures a copy of the image can open in earlier versions of Photoshop.

You also want to set this option to Always or Ask if you want to open your .PSD in non-Adobe programs. If you want to use the Edit in Photoshop option from Lightroom, you also need to enable this feature.

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