Surprisingly jaggy output - am I doing something wrong?

Question

Look at these examples:
http://www.reesweb.com/samples/jaggy/
The first is a screencap of the fullscreen view in Aperture. The image is a 1728x1153 crop from an 8mp original. Not a great picture, but it looks OK.
The second is that same version exported as a full-quality jpeg set to "fit within 1280 x 1280." Jaggy!
What's going on here? The results are so bad that I must be doing something wrong. All I'm doing is selecting the 1728 version, choosing export version, and selecting the export option (JPEG - fit within 1280 x 1280).
The third image, btw, was produced by exporting the cropped original as PSD and doing the resize/jpeg encoding in Photoshop. Looks much better.
And thoughts about what's going on here?
Will
Dual Core 2.3GHz PowerMac G5, 2GB RAM, GeForce 6600   Mac OS X (10.4.6)   Canon Digital Rebel XT, Edirol UA-5

Answer 1

OK, it looks like the jagginess only happens when I set the export DPI to 300 with my particular original and export sizes. When I set it to 72, the exported version looks fine and is not jaggy.
I see the same behavior for jpgs, tiffs, and psd.
I also tried with some small sizes and found something else. When you export to "fit within 320 x 320" and you choose 72 DPI, the image gets sharpened. If you choose 300 DPI the image is not sharpened. Strange. Maybe there's a reason for that I don't get.
This is what I'm seeing for all export types (jpgs, tiffs, etc) starting with a 1728 x 1153 image:
320 x 320, 72 DPI: sharpened nicely
320 x 320, 300 DPI: not sharpened, soft but no jaggies
640 x 640, 72 DPI: sharpened nicely
640 x 640, 300 DPI: not sharpened, soft but no jaggies
800 x 800, 72 DPI: sharpened nicely
800 x 800, 300 DPI: not sharpened, soft but no jaggies
1024 x 1024, 72 DPI: sharpened nicely
1024 x 1024, 300 DPI: sharpened but with nasty jaggies
1280 x 1280, 72 DPI: sharpened nicely
1280 x 1280, 300 DPI: sharpened but with nasty jaggies
Original Size, 72 DPI: It doesn't look sharpened at all, no jaggies
Original Size, 300 DPI: It doesn't look sharpened at all, no jaggies
50% Size, 72 DPI: sharpened nicely
50% Size, 300 DPI: not sharpened, soft but no jaggies
25% Size, 72 DPI: sharpened nicely
25% Size, 300 DPI: not sharpened, soft but no jaggies
And more data, but this time starting with a 3456 x 2298 (digital rebel default) image:
1280 x 1280, 72 DPI: sharpened nicely (some images a bit too much)
1280 x 1280, 300 DPI: not sharpened, soft but no jaggies
So it looks like the combination of using 300 DPI and certain original and export sizes can result in some nastiness.
It is also interesting to note that in most situations, using 300 DPI turns off or greatly reduces the automatic export sharpening. Juicy info for those looking for ways to circumvent that feature.
Will
Dual Core 2.3GHz PowerMac G5, 2GB RAM, GeForce 6600   Mac OS X (10.4.6)   Canon Digital Rebel XT, Edirol UA-5