PNG Photo touched up, exported file smaller in size?

[default071kwflvcvwh]

Hello, I have discovered an unusual trend, and I am unsure what is causing it.  I have scannned some old print photos into my computer at 300 ppi and PNG file type.  I then create a photoshop project, selecting the same 300 ppi resolution, and a canvas larger than the photo.   I import the photo, trim the excess canvas, and some do very basic color/contast adjustments and spot healing.  I then go to export it as a PNG file, and notice the file size is smaller. 

I checked and the dimensions of the original and the altered photo after being exported, and they remain identical (1800x1200).  It is not an overly substatial difference, but for example, I will see the original file 2.4 mb, and the touched up photo 2.0 mb.  Obviously I am not attempting to create super high quality photos, but I am very curious what is causing this.  If a photo remains 1800x1200 at 300 dpi and same PNG file type, I would think that any touch ups would result in an increase in size.  Any thoughts? TY!

[Mylenium · Accepted solution]

Nothing unusual or magical going on. It's simply how PNG compression works. Getting rid of dirt, frizzled edges and other stuff would in effect mean that you have larger areas with consistent coloring that simply compress better. You have your logic backwards there. It's exactly the other way around. Editing does not necessarily increase file size, not with PNGs, not with a lot of other LZW/ RLE compressed formats like some TIFF flavors, TGA and so on.

Mylenium

[default071kwflvcvwh]

Thank you, that was sorta what I was leaning toward the more I thought about it.  Thank you for the information!

[c.pfaffenbichler]

»I then create a photoshop project, selecting the same 300 ppi resolution, and a canvas larger than the photo.«

Would opening the png with Photoshop not achieve pretty much the same thing? 

[D Fosse]

Right. In Photoshop, there is no need to create a "project" and then "import". You just open the file, done. That's it. Just open the PNG.

I don't know why so many people do this, probably because they're used to it from other software.

And again, nothing unusual about the sizes. PNG uses compression to reduce the size on disk. Not as aggressive or destructive as jpeg, but still.