Yeah you'd think so, but I don't think it's true. Going by body weight goes really well for some drugs... but serotonin receptors like 5ht2a have a lot to do with the higher brain which is more about thinking and the most conscious stuff - maybe that's why it is more individually variable - many other things in your body develop much more independently of the sort of conscious mind you have?
Even if that's not true, gaining or losing weight won't make you gain or lose brain mass - with alcohol and plenty of other drugs it is important how it spreads across the body, but I don't think absorption and distribution of psychedelics is quite like that. In any case, I'm betting it has to do with either of those things.