Because the mirrorless cameras can have different sensor sizes, they may also require different lenses to get the same angle of view from the same place. For example, if you and I were standing next to one another and I was using an m4/3 camera with a 150mm lens, if you were photographing with a Sony E-mount (APS-C) camera you would need a 200mm lens to get the same image.
Note that manufacturers do not tend to use horizontal angle of view in their specifications as I do here. Instead, camera makers almost always use diagonal angle of view (angle across the diagonal of the capture area). That's not the way photographers think, though. Anyone can figure out what 90 degrees horizontal is (about an 18mm FX lens), but I don't think there are many that can figure out how to apply 100 degree diagonal, especially when we have differing aspect ratios (3:2, 4:3, etc.).
