One thing that has always amazed me is the tiny size of the Voigtlander 35mm f1.4 and the similar M mount Leica 35mm Summilux. These lenses, for how much light they let in, are physically very small. Compared to my Sigma 35mm f1.4 it is probably less than one third the size.
I have a theory that a large distance from lens flange to sensor / film plane to accommodate a mirror box leads to both a loss it light levels, and also of image quality. If you consider the reverse application, a projector, image quality and light levels do deteriorate the further away the place of projection.
In this case, as the Sigma is a much more modern design, image quality is significantly better than the Voigtlander; though I would be interested to see how it compares to the expensive Leica 35mm Summilux ASPH.
Here's a comparison between the two lenses.