Well the enormous sins are listed for Muslims, and those include drinking alcohol, not praying everyday (except when you are unable and you make those ones up), not fasting during Ramadan if you are healthy and able to, and not going on Hajj if you have enough money and time to do so (basically for the incredibly rich people and retired folks who have money saved up).
These are considered like HUMONGOUS sins, that should be avoided at all costs, but since smoking weed is not listed specifically as an enormous sin, then it is considered a smaller sin, of course it should be avoided if it is for pleasure, but it is just like i said something comparable to a girl not wearing a scarf. Not wearing a scarf is not specifically mentioned as an enormous sin, so it is a smaller sin.
It is also haram to cause yourself pain, so that explains why tattoo's are sins, and also why it is bad for you not to take anything to ease pain when you hurt yourself on accident like breaking a bone or whatever. That explains why smoking only enough weed to ease extreme pain is allowed and not considered a sin, as long as you are not smoking it to give yourself a "high" and only to avoid constant pain.
Some even suggest that weed should be used, since narcotics that are given over the counter are highly addictive (vicodin, morphine, oxycodone, etc.) and can make the person go into more pain without them, therefore getting closer to a sin. Since weed is not physically addictive, it is regarded in some educated Muslims eyes as a "better" alternative.