Resizing functionality in PS (2020)

Conrad,

My belated thanks for your reply. 

Your answer helped me understand the diference. 

r.

rivalg wrote:

I wonder why is the reported size so different in PS and Apple. 

Photoshop reports the true uncompressed file size of an image; in other words, the data size that results from doing the math on, for example, bits per pixel * the number of pixels * the number of channels. That is usually a much larger number than what you see on the desktop, because…

The Mac Finder (or Windows Explorer) desktop reports the size of the file resulting from both the file format and the amount of compression. This is the reason we don’t compare “quality” using file size: The same 300 x 200 pixel image could have several very different file sizes on the desktop depending on which file format they were saved in (JPEG? PSD? TIFF?), the bits per channel of the file (8? 16? 32?), whether high or low compression was used, etc. So file size is an extremely unreliable way to compare image quality. Much better to compare using pixel dimensions (e.g., 300 x 200 px vs 3000 x 2000 px).

If you want more details about that, you can read the following article (disclaimer: I wrote it):

Know Your Photoshop File Sizes

This is all about pixels. Watch the pixels. Forget ppi or file size on disk.

An image is measured in how many pixels. Increasing pixel size will bloat file size, but do nothing for quality. On the contrary, the result will usually look a whole lot worse. Don't do it, don't even think about it.

The jpeg file format uses very aggressive data compression to reduce storage size on disk. It's just packaging on disk. When the file is reopened, the data are decompressed and the file size returns. Hence the apparent difference (which isn't real).

Lots of people misunderstand ppi. It's not a native property of the file. It's an instruction about pixel density in print, determining how big the available pixels will print on paper. It's still just pixels. To help understand, read it absolutely literally, word for word: pixels per inch. It means exactly what it says, no more, no less.