You wouldn't forget your past life because of brain size as a baby, the theory of memory relying on brain size was disproven many decades ago. There are two possibilities as to why we don't remember our past lives: 1 is that it never happened and heaven is real as well as hell, the other possibility is that we're not supposed to remember them in our brain's memory because those memories are in the soul. If you think about it the brain remembers and responds to things that effect your current shell, the soul is the ONLY thing you still have from your past life and thus the soul would be where those memories are.
I believe in both theories, personally. I think there is a heaven and hell for people who don't want to be reincarnated, aren't supposed to be reincarnated, or for people who have served their time in this dimension. If the soul can retain memory then you should be able to access it through meditation or, against my advice, through hypnosis. You can't, however, set something up so that in your next life you will be born with the memories of your current life because in your next life (unless you are some kind of science experiment that happened to have the brain of your current shell) you will have a new brain which will hold new memories that will be for your new shell.