Honestly not being pedantic, what do you mean by "exponentially" in this case? Usually it implies a rate of growth though some people, increasingly, seem to use it just to mean "a lot".
Surface contamination is deadly, particularly to bird/shore life, but dispersed oil is deadly throughout the water column (until it resurfaces, at which point it becomes deadly to birds again). There's relatively little life at the surface compared to the pelagic zone, so the impact of dispersants doesn't merely multiply the effect of the oil by the depth of the water, but actually moves that oil into the regions where life is densest.