So first you develop your game for Windows on DirectX, and than you make a port to OSX/iOS/Android using OpenGL, and than if you're lucky you make a port to XBox and PS4, maybe WiiU. Graphics pipeline rendering code is really small and simple part of any game, usually in big games you do have different render code for different GPU series or/and platforms, but it's much smaller in comparison with everything else.

What's big and bulky - shaders themselves, but there are solutions to translate them between different available languages.