Game tries to cover all the space available in the window
to show you as much details as it’s possible.
When you use the scaling ability of your browser
or [+/-] buttons in the Gameroom,
you actually change the size of the others page items,
like chat and right panel, not the game size.
In this case the game frame will be changed
in opposite way to fit all the rest space.
You can switch off this option, with the button [<=>] at the top.