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.