In my previous post on zooomr, I mentioned there was a difference in the level of compression used by flickr and zooomr. I now know that is not the case, the difference is that if you upload a standard size (ie 1024px x 768px) to flickr, flickr will not reprocessed the image and will use the image as you uploaded it. So if you uploaded a 200k image at 1024px x 768px, the image available at the large size will be 200k.
Zooomr on the other hand does reprocess the photo, so if you upload a 200k image at 1024px x 768px, the image available at the large size will probably be around 600k and because it has gone through another round of compression it will be of lesser quality.
Note I have yet to see what happens if I upload a high quaility image to flickr, say a 1Mb 1024px by 768px and see if that is reprocessed.