I know for camera sensors, for example a micro 4/3 sensor is less sensitive to light than a full frame (35mm equivalent) sensor. At the same aperture and shutter speed, a full frame sensor is capable of much better low light performance because it takes in more light.
How about medium format, is it more light sensitive due to having a bigger "sensor" or "contact area"? For example, a 75mm 4.5 medium format lens, does it take in the same light at 4.5 as a 75mm 4.5 35mm film camera lens? Or more light?
Is there some sort of "equivalency" in the numbering or is there any difference in metering here to be aware of...
Speaking generally, you need to be careful when you are trying to find equivalencies between film and digital. There are lots of them, but not everything is equivalent.
As has been posted above, a smaller digital sensor isn't less sensitive than a larger one, it just doesn't give quite as nice results when the amplification is turned up in order to deal with low light levels.
For digital, low light and high ISO (which means amplification) results in noise.
For film, low light and high ISO (which means "faster" film) results in grain.
If your piece of film is bigger, you don't have to enlarge it as much, so the grain isn't as obvious in the final print or screen image.
The use of f/stops does a great job of negating the differences between formats. In essence, they refer to a ratio, and the ratio behaves the same with smaller and larger formats.
A better example: Assume you have a 135 format (aka "35mm") film camera and a 6x4.5 format medium format film camera. Assume as well that your subject fits perfectly in a 4/3 aspect ratio (meaning it will fit perfectly in 6x4.5 negative and can easily be cropped to 32mm x 24mm from the normal 3:2 aspect ratio 135 negative).
At f/5.6, a 40mm mm lens on the 135 format camera will (once cropped accordingly) give very similar results for both the image captured on film and the exposure of the film as the result obtained at f/5.6 with a 75mm lens on the 6x4.5 format medium format film camera.
I refer to a 40mm lens for 135 film and 75mm lens for 6x4.5 medium format film because each of those focal lengths correspond to the diagonal of the image area used for that aspect ratio on the film.