[bl]

I like your gist of putting the hypothesis to test, but there are serious practical factors that would prevent your proposed experiments. Small- and medium-sized mammals (e.g., rodents and primates, respectively) are somewhat convenient for experimenting in that they can be housed/fed humanely and fit into an fMRI machine whose aperture is ~0.5 meters in diameter. I do not see how one could reasonably do the same for elephants and whales.

But let's do a thought experiment and see if we can reasonably dispense with the need for experiment itself. The article (via the researchers' statements) zoomed past a detail: cortical surface area is much more indicative of neural processing capability than gross brain volume. In many contexts, a neuroscientist might use "brain size" as shorthand for cortical surface area. Also consider that more "advanced" mammals tend to have more convoluted cortexes, thus larger cortical surface areas. So it's quite possible for a large mammal's (whale's or elephant's) brain to be volumetrically larger than a human's, but to have relatively smaller surface area because it is less convoluted.

In the event that we could actually accomplish such a comparative study as you propose, we'd probably find that "tethering" does not monotonically increase with surface area. Then we'd determine that the authors' hypothesis is overly simplified.