The critical question is what concentration of drug is achieved at the point that it has its effect. Usually, the body has no special mechanism for transporting a drug there. Logically, dumping the same amount of anything into different volumes of the same liquid will get you different concentrations.
That more weight you have, the lower the concentration you get for a given dose. As an extreme example:
The 430 pound Mountain from Game of Thrones is going to need more of almost any drug you can think of to get a desire effect than say, the proverbial 98 pound weakling. How much more is complex - just because he is four times bigger does not mean he's going to need four times as much, the relative size of the brain in both people is going to be close. How fast his body gets rid of it isn't going to be in exact proportion to his weight either. Adding 40% of your body weight of fat or muscle does not add 40% more Kidney or Liver (the organs most commonly associated with metabolizing and excreting drugs)
On a professional level I am compelled to recommend that you never magic mushrooms - they and other illegal drugs are illegal because the chance of harm far outweighs any potential medial benefit.
Moreover, you can never tell how potent something like that is without laboratory testing. Overdoses often happen because people get used to a certain among of something, not knowing it is of low potency; then get a batch with much higher potency and use the same amount - which turns out to be far too much.
