> Are you saying increasing the brain capacity for a species other than ourselves and potentially enabling them to become (undeniably) sentient is morally wrong?
Yes. I would call it extremely irresponsible -- on the same level as having a child you know you will be unable to feed, so the child will starve and probably die. (With the same excuse: but wouldn't the child be happy that they existed at all?)
We have no idea how different species would think and feel. If we imagine them as "exactly like humans, only with more fur", well, who knows, it might turn out to be true; but if it is not -- if the difference between us and them is greater than the difference between any two humans, just like with the biological differences -- then we might create another intelligent species with incompatible values. Where "incompatible values" could mean that a reality that makes us happy would make them unhappy, and vice versa, so there is no outcome that wouldn't require lot of suffering.
The assumption that they would be "exactly like humans, only with more fur" makes sense from a Cartesian, dualistic perspective. Same souls, in different bodies, duh. But if you think that mind is a product of body, and tiny changes in the body can have dramatic impact on how the mind works, e.g. having more or less of some hormone can profoundly change your personality; then minds of different species will probably be quite different.
Then there is this practical problem, that we can't even take care of our fellow humans. People are still starving, dying in wars and genocides, etc. Even in first world we have the problem of poverty, racism, whatever.
Now imagine that into this world you introduce a new intelligent species, which might be incapable of getting a job, dealing with bureaucracy, or just interacting with majority humans in ways that humans consider acceptable. Because this society is designed for humans, it might be impossible to navigate for them. Then what? Keep them in... a big ZOO, forever? Tell them the same fate awaits their kids, and the kids of their kids, until perhaps one day humans get bored of them and decide to exterminate them? Or let them out, and then put them in prisons, because they may be unable to abstain from behaviors that are considered criminal by us? Require them to ignore their own insticts and live accordingly to our instincts, and then punish them if they are unable to comply?
Or are we going to give them an island where they can create a civilization according to their own norms? And ignore if they decide to do things that are abhorrent to us but normal for them? Are we going to trade with them? According to their norms or ours?
The only reason this works in some sci-fi books is that the other species are "exactly like humans, only with more fur". It may not work if they are fundamentally psychologically different.