Actually, liquid oxygen is almost 40% denser than kerosene (1.14 vs 0.82 g/cm^2). And this is the reason most rockets [1] put oxygen on top/forward: they want the center of gravity to be as far forward as possible, ahead of the aerodynamic pressure center, so that the rocket will be as stable as possible when ascending through the atmosphere. Just as we would put a weight on the front of the arrow, and feathers in the back, to make it more stable.

For the vertical landing, this would seem to work against them, but I guess the mostly empty tanks don't dominate the considerations. Falcon has oxidizer on top in the conventional manner, as far as I know (they mention LOX feed lines going through the fuel tank).

[1] The N1 rocket had oxidizer below the fuel in every stage, apparently because they wished to use spherical tanks, and this allowed packing them in a conical shape - the oxidizer tanks are much larger.