If the two objects are close to the same mass, i.e. car vs. car, the energy transfer will be reduced by up to 75%, but otherwise the mass of the larger object is immaterial. That is, a car-car collision at 60 MPH does equal damage as a bus-car collision at 30 MPH.
Extreme example: a 1000 ton freight train "hitting" you at 0.1km/h it's not doing the same damage as a 1000kg car hitting you at 100km/h.
Note that this implies that unless there are other dangerous road conditions (fog, ice, etc), the safest speed is "the same speed as everybody else" so the delta-v is minimized.
Fun note for HN: he had software built for DOS that I helped him get running in dosbox so he could run it on a modern computer. It would reconstruct the motion of the vehicles from the final resting positions and the depth of the dents. working backwards from the implied kinetic energy. Apparently the DOS version was a port of his original FORTRAN source... on punch cards.
The potential damage may be greater, but significantly lower speeds creates a much greater risk of some accident. All things considered, then, it's not clear what's worse at the bottom line.
The chance of an accident is greatly increased when vehicles are traveling at very different speeds. See, for example, http://priceonomics.com/is-every-speed-limit-too-low/
f = ma. The bus is considerably more dangerous than a car, even while traveling at a significantly lower speed. As the parent comment noted, there are certainly some aggressive bus drivers out there as well.