Mars at Opposition - Earth at Transitition
If you weren't using a telephoto lens or a telescope or something else to magnify the image, then it probably wasn't the actual disc you were resolving. It was probably just the excess light from Mars "bleeding" onto adjacent detectors on the surface of the camera's CCD. Or maybe the camera had trouble focusing?
For comparison, I have a 2 megapixel camera with 3x optical zoom, and when I hook it up to my 3.5" newtonian telescope, the disc is still tiny. You really need a telescope or a serious telephoto lens to be able to resolve the disc.