The Medium Format sensors are very far behind Canon and Nikon in technology, especially when it comes to low light performance (AKA "read noise"). Their new cameras match the performance of what Canon was doing about 5 years ago. That said, even with lower technology, they do have a pretty big size advantage, so that helps to balance things out. Depending on what ISO you are looking at, the DLSRs are either "same" (e.g. ISO 400) or much, much better (ISO 12800).