Convert allocates four-times more memory it needs?

Hi.

I'sent this to magick-users, but maybe this is better place.

It seems convert allocates four times more memory it needs (and kills my
gallery server). I've got such image:
$ identify dsc_6436_fx2.jpeg
dsc_6436_fx2.jpeg JPEG 4178x2774 4178x2774+0+0 8-bit DirectClass 946kb

The Gallery2 tries to downscale it using command specified bellow, I've
just added "-debug all" parameter and redirected output.

$ convert -debug all -quality 85 -size 800x600 -geometry 800x600 dsc_6436_fx2.jpeg jpg:/dev/null | grep heap
  open /tmp/dsc_6436_fx2.jpeg[0] (heap memory, 8356x5548 707.4mb)
                                               ^^^^^^^^^^^^^^^^^
  open /tmp/dsc_6436_fx2.jpeg[0] (heap memory, 800x531 6.482mb)
  open /tmp/dsc_6436_fx2.jpeg[0] (heap memory, 8356x531 67.7mb)

Wy does it allocate memory for image that is four times larger than it
really is?

If I ommit the "-geometry 800x600" parameter, it allocates reasonable
amount of memory:
$ convert -debug all -quality 85 -resize 800x600 /tmp/dsc_6436_fx2.jpeg jpg:/dev/null | grep heap
  open /tmp/dsc_6436_fx2.jpeg[0] (heap memory, 4178x2774 176.8mb)
                                               ^^^^^^^^^^^^^^^^^
  open /tmp/dsc_6436_fx2.jpeg[0] (heap memory, 800x531 6.482mb)
  open /tmp/dsc_6436_fx2.jpeg[0] (heap memory, 4178x531 33.85mb)

What's going on? I haven't update Gallery2 recently, co I don't thing it
changed parameters passed to "convert". So I suspect problem with
"convert" or "libjpeg" which my distro updated recenty from 6x to 7.

Do you have any clues?

ImageMagick-6.5.3.10-3.i686
libjpeg-7-1.i686

Regards

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