When I save the same image from Photoshop CS4 and Paint.NET, I get the exact same image but different file sizes, and they look exactly the same.
Photoshop - Photoshop - 3.564 bytes - 32bit PNG with transparency, duh.
Paint.NET - Paint.NET - 704 bytes - 32bit PNG with transparency, again, duh.

Why this? Why do identical images in identical format occupy different sizes?

Edit: Does it matter that Opera Image Properties say that the Paint.NET image is 8bits but it's still NOT? Windows Explorer file properties say it's 32 bits...

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    It's probably the gray circles between the black squares. They keep shifting in and out and sometimes vanish completely depending on where I look. I think they are the reason for the extra bytes. (Joke about the optical illusion nature of these example images) – Tyler Faile Jul 5 '11 at 19:57
  • You get +1 for actually making me laugh! – Vercas Jul 5 '11 at 20:01
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    Try compressing your PNGs with this service: punypng.com They should be reduced to the same size. – Alex Jul 5 '11 at 20:17
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    Opera is correct, the smaller file really is 8-bit. It appears Paint.NET has performed an unauthorized optimization. – Mark Ransom Jul 5 '11 at 20:39
  • @Alex That thing sacrifices quality of my PNG images! – Vercas Jul 7 '11 at 11:23

It's all about how the compression settings are used (or misused, in some cases).

Photoshop is notorious for its bloated PNG output. Paint.NET simply correctly takes advantage of the compression built into the PNG spec.

There's also the possibility that other metadata may be saved into the file, resulting in size differences.

OptiPNG could probably make either file smaller. In fact, I think there's an OptiPNG Plugin for Paint.NET. (Found It!)

Wikipedia says:

Some versions of Adobe Photoshop, CorelDRAW and MS Paint provide poor PNG compression effort, further fueling the idea that PNG is larger than GIF. Many graphics programs (such as Apple's Preview software) save PNGs with large amounts of metadata and color-correction data that are generally unnecessary for Web viewing. Unoptimized PNG files from Adobe Fireworks are also notorious for this since they contain options to make the image editable in supported editors. Also CorelDRAW (at least version 11) sometimes produces PNGs which cannot be opened by Internet Explorer (versions 6–8).

Adobe Photoshop's performance on PNG files has improved in the CS Suite when using the Save For Web feature (which also allows explicit PNG/8 use).

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  • Also, incredibly useful plugin. Now I can optimize my sprites even more! – Vercas Jul 5 '11 at 20:18

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