If you change the background from DefaultDrawable to any other, the button extends to the total size.
The four upper buttons, default background.
The four lower buttons, ColorDrawable with border, GradientDrawable, BitmapDrawable and ColorDrawable without border.
Be aware that buttons use the StatelistDrawable with different backgrounds for the different states.
Attached the demo program.
As emexes already suggested, Panels are also a solution.
I use Panels instead of Buttons quite often in my programs.