First thing I would like to address is the fact that your head is certainly not shrinking! I am not a medical doctor, and I know how this board is about medical advice, but I think it is pretty safe to say that your head is not shrinking in size. I am willing to bet that your friends head is not shrinking in size either.
Marijuana is a fairly benign drug physically if you do not consider the long term effects that harsh smoking has on your lungs. Typically people will not overdose, and die while smoking this drug unless you have some sort of allergy or other adverse reaction.
I was hopelessly addicted to Marijuana for about 3 years. Although I do not believe that Marijuana will cause someone to die, or shrink their heads - I do believe that Marijuana can cause intense and severe psychological/emotional harm to the user. Marijuana is a very peculiar drug that skus reality to the point where the user can experience overblown and intense emotions. This is why paranoia often becomes a strong side-effect of this drug. Someone under the influence of THC can look at a pack of gum, and suddenly consider every step involved in the manufacture and distribution of that pack of gum, and then take it further to consider exactly how that pack of gum seemed to make its way to the users hand. Suddenly the concept of the pack of gum making some sort of epic journey from its point of orgin to your hand becomes an intense emotional experience for which the user feels the need to share with anyone that is around him/her. Yea... This is not normal. In your case, it seems that you pieced together a strand of cause/effect situations that ultimately caused the shrinking of yours, and your friends, heads.
Marijuana is a dangerous psychological drug. Its effects can cause very serious damage to the users perceptions of reality, and can ultimately cause the user to see the world differently than it really is.
This is why I had to stop using it. My emotions and perceptions became too much to handle, and it had to stop.
You are young, stop now, and move on with your life. Be happy that you caught this at such a young age. And please - understand that your head is not shrinking..