...if one is a genius, they have not developed normally. If you are really tall, you have not developed normally. If you have red hair and blue eyes, you are technically in the realm of abnormal [they are both regressive, meaning an extremely small percentage of the world's population possess both traits].

If you have a mental illness, something about you didn't develop normally- your brain. Whether inherent or by environment, or both, your normal path of development was altered somehow.

That doesn't indicate anything other than what it says: abnormal development leads to abnormal functioning and suddenly we have clusters of symptoms we categorize into mental illnesses. The RDoC [NIMH in the US] is looking into definitive ways to identify symptoms of mental illness from a biological standpoint- much of this includes functioning and development of the brain. There are a lot of specific ways that brains belonging to those who identify as having mental illness work differently. That's not saying anything demeaning or negative, it simply saying their brains have developed outside the realm of what is normally expected... which would be why certain aspects of life are more challenging.

When people have said in the past that specific people of a specific "race" had smaller brains, it was an attempt to make a connection between supposed "subhuman" traits [false] and that particular demographic.

If we choose, in this case, to completely ignore biological development as a key factor in mental illness, either on it's own or as a result of environment, it's pretty much cutting off the nose to spite the face.

And neuroplasticity, as ocdwife said, is a HUGE thing. A huge AMAZING thing- for a long time everyone was convinced that a TBI meant, no question, permanent damage and that any damage or missed developmental input meant that fate was sealed, there was no turning that around.
When I talk about brains, neuroplasticity is a given and accepted concept- for me. But not for everyone, and I get that. But some of the medications we take- some of the anticonvulsants- actually have neuroregenerative properties.

But abnormality could be anything on either end of the bell curve, and acknowledging abnormality does not endorse any kind of idea of inferiority, unless we choose to do so.

One more thing:
Any consistently applied intervention has the power to change the brain to some degree if applied long enough and in an effective manner- not just medication or the more invasive treatments. One of the challenge is applying any intervention with rigorous consistency, and also being able to determine when which interventions are most appropriate.