Comment 5 for bug 763517

those calculations are done to make the button's size resolution independent in the menus, defaulting to one button size would be bad if you increase the resolution: the buttons would need scaling.

A better approach might be remodel the UI to work with normalized coordinates going from 0 to 1 or so. With this, the resolution would not matter for the layout and your approach with the default sizes will work out.

As it stands now, I cannot see how the button default size can be moved inside the button class besides adding a static member variable. That is clutchy though.