see this article for info on Photostream
an excerpt form it
On your Mac or PC, your photos are downloaded and stored in full resolution. On your iPhone, iPad, iPod touch, and Apple TV, your Photo Stream photos are delivered in a device-optimized resolution that speeds downloads and saves storage space. While actual dimensions will vary, an optimized version of a photo taken by a standard point-and-shoot camera will have a 2048 x 1536 pixel resolution when pushed to your devices. Panoramic photos can be up to 50 pixels wide.
I uploaded photos to Photostream using iPhoto on my Mac and then went to the public photostream link to download them on the same computer. The downloaded photos onto the computer are under 1 MB; not the 8 MB the originals are in.
I read that same thing at Apple, but now I am thinking that if you publish the photos using Photostream and also choose the "public" option; then folks who click on the public link and then download the pictures onto their PC, that it actually downloads at some type of lower resolution.
I get that this happens with portable devices or Apple TV, but it appears to be happening also with downloading to a PC or Mac.
Yes, I took another look and you are correct on the pixel dimensions. I download the photo from Flickr at original size and it was 4,000 x 6,000 at a 300 DPI. The same image downloaded through iCloud Photostream was 2,304 x 1,536 at a 72 DPI. I used Preview inspector to get those stats. Note that with Flickr the file size was around 5.1 MB (originally, it was around 8 MB, it is a 24 megapixel DSLR Nikon Camera). With iCloud Photostream, the photo size downloaded to my Mac was about 800 KB.
It would be nice if there was an option in iCloud Photostream to download the original file size. Many to most times you are sharing and folks would like a high-quality original for whatever reason of one particular photo from the set.
Similar to how iPhoto keeps an original size around, it would be nice to have that option with iCloud Photostream.