I'm not sure what cubic volume vs 2D surfaces means to you, but these comparisons are by mass, not volume. Plus, anaerobic bacteria would only come up on top if you counted all of them as one species, which they are not.

Why is that group inherently more massive than waterborne bacteria?

I think the limiting factor in the amount of cellular mass that a volume can contain is probably either carbon content or nitrogen content.

Both maximize in the oceans near to shorelines, with smaller local maxima near some vents.

On land, I'd expect a gradient in both, decreasing with depth, but there's probably more CO2 100-m down in water than in rock.

The same argument could be had for fish, since they inhabit the volume of water, instead of the thin land surface of earth.