These cameras all feature a 50-megapixel sensor (probably the same Sony-made sensor), which measures 3.3x4.4cm. The smallest medium format film size is 4.5x6cm, which is quite a bit larger than the sensors found in these new digital cameras. Size-wise, the difference is significant!
But with digital photo technology, sensor size doesn't always tell the whole story. Or, at least, you have to understand what it all means and doesn't mean. The larger the sensor, the larger the light-sensitive elements can be. To squeeze 50 million light-sensitive elements (or "pixels") onto a full-frame sensor means using smaller light-sensitive elements. If the sensor is larger the pixels can be larger, too.
|Horse At Fence - Onyx, California|
Captured using a Holga 120N medium format film camera.
Resolution is resolution. 50 megapixels on a medium format sensor is the same as 50 megapixels on a full frame sensor or even a smaller sensor (if one were made). The real difference is in noise and dynamic range, and full frame sensors are nearly as good, and sometimes better than, medium format sensors in those regards.
One aspect that I haven't mentioned yet is depth-of-field, and the way lenses render images on larger sensors. There is a unique quality there, but it is subtle and probably unnoticeable by many.
To get back to the question that was asked to me, are these new cameras really medium format? Technically, yes. Anything larger than full frame (35mm) but smaller than large format (which doesn't exist in the digital world, and barely still exists in the film world) is medium format.
The real question is this: are these cameras worth the higher MSRP? Is it worth paying double or triple to get a little bit more resolution, a slight increase in high-ISO and a tiny gain in dynamic range? I can't answer that for you.
If I had money lying around, I'd love to get a medium format digital camera. But I don't so it is unlikely that I will ever shoot one. I don't feel like I'm missing out on much because cheaper gear performs very well nowadays. Yes, it would be nice to have one, but it is far from essential.