Airbags make only a marginal difference in most cases where seatbelts are worn. And I am afraid if your Disco and a Yaris collided, you are a lot better off in the Disco even if the airbags did not work. Of course, in a collision with a B-double or a bridge abutment or tree, it does not matter which you are in - you are almost certainly dead, airbags or not. Airbags extend the margin of survival a small amount beyond what seatbelts do, but the margin is small, and will cover only a relatively few accidents.
Other vehicle factors are likely to be more important, particularly those that affect the probability of an accident in the first place or its severity, but also the simple physics that in an collision between a light and heavy vehicle, there will be more damage to the small vehicle.
MUARC research shows no overall correlation between deaths/injuries and safety features, although there is a correlation between these features and deaths/injuries if you only look at accidents that have actually occurred.
John

