Brain: Brain case volume may not relate to size

A friend draws my attention to this item in Nature News:

Basically, there is apparently no necessary relationship between the size of brains and of brain casings:

You may be struck by how small the brain (yellow) is compared to the braincase (red). If the scientists are right, it’s a cautionary tale for those who would estimate the size and shape of ancient fish brains from their braincases. But perhaps, in the future, researchers will find more actual brains, and will be able to chart the evolution of these delicate organs in greater detail.

Interesting. If we assume that the universe shows evidence of design, this would be easy to explain .A fish may need a bigger brain later than it does now. So what do we do? Build a case that can house only a very small brain or one that can house a bigger brain if needed?

Oh wait. Nature is blind and mindless, right? So ...