The bigger the format, the less the depth of field for the same f value. The focal length will vary, simply because the image receiver is bigger. If you use a projector with a fixed focal length lens at home you can see this, you need to move the projector back to get a bigger image, (ie the focal length) and the image isn't as bright when you do, (which equates to the f number).
In cameras the DOF between formats is determined by the size of the glass. So a 6x6 f 4.5 might have the same DOF characteristics as an F2.8 35 mm lens. The medium format lens will have a bigger focal length number because the same size of objective lens has to be further away from the receptor, and the f number is higher because the light is spread over a larger area.