It make sense to keep the global design of the website.
In that design, game canvas are supposed to be contained within the layout of the webpage.
If the canvas was wider than the actual available size, it could visually break the design.
The arcade is being worked on by Tom anyway, at some point a new layout and less limitations should arrive.
The limitation preventing "many good games away" from the arcade, in my opinion, is the fact that you can't use third part plugins/behaviors, rather than the size of the game itself.