[D-fendr]

The more spread out the oil is

You're missing my point. It's huge, the surface, where it is spreading out, is a tiny percent.

[Wonder Warthog]

And so what??? What matters is the total emitted. The longer it takes to come to the surface, the more time it has to decrease in droplet size, dissolve in the seawater, and be eaten by bacteria. It is true that all of those processes work faster as it gets closer to the surface, and once it "does" reach the surface, it is exposed to air (more oxygen) and sunlight, which brings other degradation mechanisms into play.