file size / pixel dimensions : interchangeable?

a photoshop question from a newbie...

[mightymid]

I have a VERY basic PhotoShop question, and have been banging my head against the wall trying to find the answer on the web.

I opened a graphic in PhotoShop, went into Image Size and saw that the pixel dimensions were 500 x 375 px ... or 550k, which I assume is the file size.

However when I check the file properties in the subdirectory, the file size is substantially smaller (around 40k).

Why the difference? It seems that I'm not understanding something very fundamental about image size.

Thanks,
Mid

[choster]

The first figure is the amount of bytes of memory required to display the image on screen. This figure is determined by the pixel dimensions of the image.

The second figure is the amount of disk space required to store the image. This varies depending on the pixel dimensions of the image, the resolution ("pixel density"), the file format you use, and the composition of the image (e.g. a 4-color logo versus a complex photograph), the second number may be substantially smaller than the former.

It is the latter figure that a web designer should concern him/herself with.

[mightymid]

Gotcha. That makes sense. Thanks very much, Choster.