Presumably, as a whale or seal dives, its lungs get compressed by the increasing water pressure, and it gets less buoyant.
Under this model, for a given amount of air taken in at the surface, the animal has a very specific depth D at which it is neutrally buoyant. Any deeper, and the lungs shrink, making it negatively buoyant. Any shallower, and the lungs expand, making it positively buoyant.
In other words, until it reaches depth D, the animal is positively buoyant. To get to depth D, the animal would have to do a lot of work swimming down, fighting against its own floatiness.
The amount of air that keeps you neutrally buoyant at 10 meters will inflate to twice the volume once you surface, keeping you pinned there. Speaking as a scuba diver, I can say that it would take a lot of work to dive back down to 10 meters. (We deflate our air vests at the surface and re-inflate at depth from our incompressible tanks, something animals don't have.)
For very deep divers like sperm whales, swimming down against positive buoyancy seems like a huge energy waste, the kind that one might expect wouldn't be tolerated long by evolution.
So do marine mammals do anything to compensate for the effect of fluctuating lung size on their buoyancy? One guess is that they actually exhale fully before diving, thus making lung buoyancy a small player in the overall body buoyancy. Another guess is that they compress the air with their chest muscles at the surface in order to sink. These are just guesses; I'd love to hear the real story.