Like I said:
If this doesn't help, could you share your capx, for this problem should not exist if your game is set up correctly..
I don't know about cocoon.js the exporter, because it has been depricated for a while now..
If the game is working fullscreen on preview, with all different browser-sizes, it probably is related to that..
If it is, unfortunately, it is not a Construct2 issue..