Yep, you're right. I keep forgetting, given the specs, that that Sigma is a crop sensor, especially at that price point. As mentioned previously, the Pentax 645d is a 40MP medium format body for around the same price. The sensor size difference is significant: 24mm x 16mm vs. 44mm x 33mm (Yes, if we want to be nitpicky, the 645d is still a "crop" sensor in that it has a crop factor of .787).
I'm not sure how, with that pixel pitch, this camera will have any decent low-light performance whatsoever.