If the trend appears to persist for a long time in the fossil record, this indicates that natural selection was pushing in that direction for all that time. There will be times when increased size of some organ is favoured and other times when decreased size is favoured. Most of the time, average-sized individuals will be favoured in the population and both extremes will be penalised. It is all the more intriguing to find that sometimes long, progressive trends in one direction do turn up. Even when the average gets bigger, the optimum is always one step ahead.
Prey animals evolve faster running speeds because predators do.
Consequently predators have to evolve even faster running speeds, and so on, in an escalating spiral. Co-evolution occurs whenever the environment in which creatures evolve is itself evolving. At some point in the evolution of brains, they acquired the ability to simulate models of the outside world. It may be compared to the virtual-reality software that runs on some computers. Now here is the point I want to make.