Strange result: layer offset and filesize

[poison]

Hi ^^
First I wanted to apologize for using such large files, but I wanted to demonstrate the issue with the filesize.
I have a small script that basically does the following:
convert "src.jpg" -trim "tmp.png"
convert "tmp.png" -resize '768x768>' -strip "src.png"

Whenever I use this script to crop&convert&resize images I get a really strange result when opening the file in Gimp:

Theres always this Layer offset. Here's the file this screenshot is from: http://217.28.99.42/DPKLAKT20080004.png

The second problem is that the resulting file is always extremely large, for this source file: http://217.28.99.42/DPKLAKT20080004.jpg
its ~1.5MB in size.
When opening it with Gimp and doing:
Image -> Fit Canvas to Layers and saving it the file has only ~390KB: http://217.28.99.42/DPKLAKT20080004_.png

Is there anything I'm doing wrong?
I'm using imagemagick-6.3.7_4 and gimp-2.4.2 on slackware-current.

[magick]

Add +repage to your command line to remove the page offset.

[poison]

Thank you, that works.
But what about the filesize (1.5MB vs 390KB)?
With -quality 0 its 1.5MB and with -quality 100 its 1.6MB, ~4 times the size compared to saving with Gimp.

[magick]

Add -depth 8 to your command-line to reduce the image size.