There is another way to look at this. Photoshop is a sophisticated image editor. Some edits are about changing pixels. Others apply changes via mathematical adjustments, such as levels, curves, blending different layers, etc.
To do this and avoid clipping internal values, Photoshop will import,say, a jpg into a virtual internal space. This may be 16, or 32 bit space, at the same time working in a much larger colour space such as AdobeRGB or ProPhoto.
If you open an 8 bit jpg from Lr into Ps, using user settable default values for bit depth and colour space results in a bigger virtual (ie still in memory) file size.
The size visible from within Ps reflects these settings.
Then when you wish to save the file , Photoshop will default typically to a 16 bit tiff or psd. Again, you can change these default values.
Therefore opening an 8 bit jpg and saving it as a 16 bit psd, with absolutely no edits involved will result in a psd which will be at least twice as big as the jpg , but maybe supstantially greater, depending on compression options.
Adding edits, layers, masks etc then proportionally makes the file bigger.
I was initially horrified how big a raw file in Lr became a monster Psd after a round trip to Ps. I stopped worrying about it for 2 reasons.
1. Return trips to Ps, for me, are a small percentage of my images captured.
2. Storage space has become very cost effective.
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