i dont think that weight of a person is that important per se. i mean, the substances themselves act on the brain, and that is pretty constant regardless of body size.
what i do think matters is in places where metabolism (liver for instance) gets a word in the whole process. a larger person might have a larger, or more active liver and maybe metabolize the substance more/faster and influencing the end trip.
im also thinking at alcohol for instance, larger people neer more, not because their brains get affected in a different manner, but because fat absorbes alcohol better and thus "hides" some of the substance from the brain.
there are also people with more active enzymes from breaking down substances, and other with lazyer ones...
it could be either of these factors that have a bigger influence in drug availability in the blood, rather weight.
theres also lots of factors to consider as well, especially with natural occuring drugs like mushrooms (some batches are more powerful than others and makes comparisons by weight difficult, age of batch is also important as is storage) - ive had batches that blew me away at 2.5 and sometimes 5 was only a mild trip.
regarding DXM, K is also dosed according to kg iirc, and i bet MXE might be as well if it ever were to be properly researched. i suspect this to be the case with all dissasociatives