I don't see a handy possession vs distribution summary, but look here for the types of cases that get prosecuted for trafficking:
http://isb.ussc.gov/content/pentaho-cdf/RenderXCDF?solution=...
The typical federal marijuana trafficking prosecution involves possession of 10 kilos (the small number of cases involving less than that averaged 4 kilos). The numbers for crack and meth are smaller, but there is still a pretty clear line at 'dozens of doses'.
I'm not sure I understand it correctly, but this graph seems to say that the majority of people sentenced under drug guidelines are convicted of some other crime. So apparently they are committing other crimes while in possession of drugs:
http://isb.ussc.gov/content/pentaho-cdf/RenderXCDF?solution=...
And then the majority of "pure drug cases" actually do involve trafficking, which the data above indicates usually involves substantial quantities.
Nothing I've said is an argument in favor of the war on drugs.