What would happen/Is it possible

[Aoi Magi]

Brain size isn't the only factor effecting intelligence, and I personally feel, after an optimal threshold, and increased mass would be counter-productive. You see in an infant the brain will develop through synaptogenesis, when the synaptic connection initially form, however recognizable intelligence emerges in the later stages of brain development when synaptinc pruning occurs where useless connections are dropped-off and get rewired to make things more efficient. Genes such as HMGA2 might play an important role here.

Think of it like this, the more data you put in your mobile memory, the slower it gets, because processing the increased data takes longer. When extended to the brain, after a certain limit, I feel the brain would start losing it's efficiency as well.