Using Windows 10 and Photoshop 21.0 I created a document 6,325 pixels square. That's 40 million pixels.
I set the File Handling preferences for PSD to disable compression and maximize compatibility. I double clicked the background layer to make it "Layer 0". I filled the image with neutral gray and added fine noise to make any compression minimal. These steps should make the PSD as large as possible.
I saved this PSD in 16 bit and got a size on disk of 229.55 MB. I re-loaded it into Photoshop and read the Document size inside Photoshop as 114.5M.
When I set PSD prefs back to compression with no compatibility and flatten to have a true 1 layer background, I get a size on disk of 114.48 MB (smallest possible).
So, I have no idea how or why you got a 938MB file out of a 40 million pixel image, regardless of where you are reading those numbers. One place to look is in the Channels Pallet. Extra channels (masks) can increase files size a lot. But it would take a lot of channels to get to 938MB.
PSD file sizes are affected by the File/Preferences/File Handling options in Photoshop. Compression makes smaller files. Compatibility makes files larger. Compatibility just sticks a flattened jpeg version inside the PSD file so non-Adobe programs can show the contents. It can almost double file sizes.
Hope this helps, and sorry you got caught in the never ending grade school fight between two of our most famous bullies.