Just discovered this too - a real bummer
I do a lot of photography on my iPhone 4s. I like to edit those pictures on the bigger screen of the iPad. The easiest way to have my images appear on the iPad for editing is the magic of iClould and Photostream.
But with this automatic resizing, I am losing valuable pixels that lower the quality of the final image, especially if you make prints from photos. Maybe prints are an anachronism in Cupertino, but I still like them.
I guess I will need a different strategy to port the images over from my iPhone to the iPad. Lets see ....I'll let Photostream copy them full size to my laptop, then I'll create folders, set up the sync, and get them on the iPad that way. Way too many steps and not a very elegant solution.
A setting that allows you to select full size photostreaming would be much appreciated!