I totally understand. BUT ... Think on this:

When adding the game on your game page you have the normal size first. All users which are playing in normal size and do not want to play in fullscreen have now the problem. Eg.: a match 3 game. If you click on the right corner diamond you dirctly zoom the game. The zoom function should always be in seperate frame called settings. Or on the start page where you have enough space for this. Not in the play area overall. If anyone has a better solution lets go :-)