IIRC (David should be able to confirm) the mass of an object is proportional to its size. Therefore bigger objects require more force. If you're using a custom collision mask, it depends on the area inside the polygon.
I'm not sure this is the best way to do it, actually. It doesn't seem to make sense saying "Mass = 1" then having that change if you stretch the object...
And yes, if you use an ellipse, it'll have the same with/height as the bounding box.