The way I see it, you are not born with a 100% brain and then only use 10% of it while the remaining 90% goes to waste.
I imagine that you are born with a standard size, normal brain and then, through learning, schooling and experience, it gradually increases the internal links and neurons (?) etc. to accommodate and record the information/data acquired over the years. i.e. the more you study, the more links and connections are made to the memory and the more you know.
As you get older, these links are inclined to fail through lack of use and bodily nourishment which, later on, can be the start of Alzheimer's/Dementia, etc.
I suppose the brain can be likened to a flexible hard drive, the actual file/info capacity depending upon the user/operator whilst the registry (natural bodily functions) runs automatically.
All of the above could be rubbish but at least it is something else to read on a Sunday evening.