png convert again in irfanview get smaller size!

[Albireo]

Hello!

Converts a jpg image (big size) to a PNG image (120x120 pixels) with the following windows command:

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convert -define jpeg:size=120x120 c:\temp\pic\4613100.jpg -thumbnail 120x120^^ -gravity center -extent 120x120  c:\temp\thumb.png

(it work's)
The size of the Image c:\temp\thumb.png is 19500 byte.

If I convert the image again with IrfanView (following command)

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i_view32 c:\temp\thumb.png /convert=c:\temp\thumb1.png

The new size is only 1820byte

Why? (I can´t se the differences in the images)
What information has been lost?
Can I do the same thing, but only in ImageMagick (with the same result)?

//Jan

[fmw42]

different png optimizations.

see
http://www.imagemagick.org/Usage/formats/#png
http://www.imagemagick.org/Usage/formats/#png_non-im