For 3D rendering, yeah, the geometry and textures are the same, so you re-use them. But you get double the output. And it's not just RGBA data, generally you o

For 3D rendering, yeah, the geometry and textures are the same, so you re-use them. But you get double the output. And it's not just RGBA data, generally you output AOVs of Z depth, UV coords, Normals, XYZ world position pass, motion vectors (forwards and backwards), specular contribution, reflection, ambient occlusion, object ID, material ID, etc, etc. And that's stored normally as EXR files (16 or 32-bit float), which can be compressed, but for 4K res is generally around 40MB per frame (RGBA). For a full set of AOVs, you can be talking 640 MB per frame, x 2 for stereo which adds up to a lot. Assuming a 2 hour movie, at 24 fps, that's around 172k frames. If it's not a full CG film, if you're lucky (i.e. no manual 3D conversion) you've got double the video footage as well. These all need to be composited together, producing twice as much on the output of that.