The smaller the transaction amount, the larger chance that the dealer is going to have an addiction of their own. It’s easier for one wholesaler to sell to an addict (or several) that distributes small amounts to a bunch of other addicts, vs taking on all the volume yourself. Addicts usually know other addicts and have a strong incentive to sell, they need cash every single day.

I doubt you’ll find a source, but the number of users distributing small amounts is probably larger than the amount of people pushing weight (who usually don’t use)

This is based of my experience buying heroin in a mid-sized US metro area, fortunately far in my past. One without an open-air drug market, so deals were done in cars or alleys.