Fullscreen has many problems on a browser free flash game.
The First is performance, flash player 10 is not great when it comes to drawing the screen, so fullscreen with such large images would kill most computers, maybe with flash player 11 things will change
The second is sponsors. Most game sites are designed for games below 800px width (most even lower), they are designed this way to fit their ads, so when they see a very big screen game they one of two things : Don't host the game or resize it to fit killing the game visuals in the process. (if you search the web you will see many sites hosting a resized KR and you will know what i mean).
We would love to have a KR HD fullscreen, but i guess that will be a standalone game instead of a browser one.