As far as i understand it hals brain file (mine is a custom brain with name "Angela " ) grows larger as it learns but it isn't as huge as you think , i have worked on mine for years now and it's only about 40? mega bytes which by pc standards is very low unless you have a tinker toy for a computer lol!
The hal brain and size can be seen in the hal program folder placing the curser arrow over it will show the size .
I know about what mine is as i always back my custom brain up as much as possible !
Hope this helps to answer your question .