One species I have read about is carp whose growth is limited only by food supply and space. One can actually, because their skin is almost transparent, see their brains and it sure does look like larger carp indeed have proportionately large brains. If this is so, are their behavioral consequences and do such carp recover from brain injury?
Your question falls into an area of study known as allometry.
It is generally believed that brain size must increase as body size increases1,2. This relationship has been demonstrated in fish 3,4, including carp. This phenomenon is also seen in other (non-mammalian) vertebrates including the Nile Crocodile (Crocodylus niloticus)5.
This continued growth is correlated with the ability to regenerate after brain injury2.
Kaslin, J., Ganz, J., & Brand, M. (2008). Proliferation, neurogenesis and regeneration in the non-mammalian vertebrate brain. Philosophical Transactions of the Royal Society B: Biological Sciences, 363(1489), 101-122.
Brandstätter, R., & Kotrschal, K. (1990). Brain growth patterns in four European cyprinid fish species (Cyprinidae, Teleostei): roach (Rutilus rutilus), bream (Abramis brama), common carp (Cyprinus carpio) and sabre carp (Pelecus cultratus). Brain, behavior and evolution, 35(4), 195-211.
Ngwenya, A., Patzke, N., Spocter, M. A., Kruger, J. L., Dell, L. A., Chawana, R., ... & Manger, P. R. (2013). The continuously growing central nervous system of the Nile crocodile (Crocodylus niloticus). The Anatomical Record, 296(10), 1489-1500.