Question: Does the size of buddhi always increases in a series of incarnations?

Answer: Not necessarily. Degradation is also possible. It is mentioned in the Bhagavad Gita, in particular. Krishna says that if man gives way to anger, he loses memory of his previous spiritual achievements, and losing this memory he also loses the energy of consciousness. In other words, not only crystallization, accumulation of “mass” of energy of consciousness is possible, but also de-crystallization.