What makes the effects of some drugs more dependent on brain weight than body weight?

Factual Questions
1

I use “drugs” in the sense of substances other than food which affects the body or mind.
Typically, when people are determining the proper dosage for a drug, they calculate it according to bodyweight. Yet, for other substances, the effects of the drug seem to vary little from one bodyweight to another. Why is this?
Is it chiefly a question of the drug being a psychoactive drug or not? Even among psychoactive drugs, are there some drugs whose effects vary little from one bodyweight to another?

2

Most if not all drug effects vary according to body weight. They also vary from person to person for a myriad of other reasons, many not understood at all.
However at this point in time , I’m not aware of any drugs dosed based on “brain weight”.
OK maybe the zombie vaccine that is currently in stage 6 (top secret) development.

3

Children’s weight is almost always taken into account and dose is calculated at mg/kg . Adults…it depends on the drug and the route to be administered. Oral medicines, particularly pills, are made in standard sizes. Sometimes only one or two. IV medicines which are necessarily liquid, are based on weight much more often.
Sorry not to address the psychotropic aspect.

4

Yet another basis for dosing: Body surface area - Wikipedia

5

That’s a good one. It likely ends up being a good proxy for the fact that many of the processes which occur in the body are a function of surface area rather than volume. E.g.: if you double the dimensions of a muscle, you will increase its volume (and pretty much also its weight) by a factor of 8 but its sectional area will increase by a factor of 4 and it’s the sectional area that is causal in terms of strength. Same thing for lungs taking in oxygen, kidneys filtering blood, intestines leeching molecules from what was ingested.

I was thinking of psychedelics, most especially LSD, DMT and MDMA, in contradistinction with alcohol, cocaine, pot. This account suggests that brain size or metabolic rate may be more pertinent for some drugs: Erowid LSD (Acid) Vault : LSD Related Death of Elephant in 1962