It all depends on the "accident" (which is a bad term anyway)
Remember the lighter car will be easier to stop, slow down or have change direction than the larger car so maybe it'll avoid the accident all together. This is something that's often overlooked by the big 4wd fans with their long-travel suspension, high c-of-g and ballon tyres, all of which work against avoiding the accident in the first place. This is always the best option
However, if a collision is going to happen and it's with another car your better off being in the heavier one. If it's with something that won't move (solid wall, large tree) the less energy that needs to be absorbed by the crumple zone the better.
Thus lighter (but larger) car is better than heavier (but small) car.
Airbags are the last line of defense to cushion the body parts against hitting something hard.