You CAN just make one large button size and get away with it, but thats not best practice.. You should have different sized images for different resolutions.
From Android: Although the system performs scaling and resizing to make your application work on different screens, you should make the effort to optimize your application for different screen sizes and densities. In doing so, you maximize the user experience for all devices and your users believe that your application was actually designed for their devices—rather than simply stretched to fit the screen on their devices.
More info on supporting different devices and best practices. Android Practices