another difference between the mafia and street gangs is that street gangs didnt start off as a criminal thing. they were mostly just little clubs that were defending there neighbourhoods, it wasnt till much later with when drugs were introduced into things that they started to organise into more of a criminal group, unlike the mafia who practically engaged in criminal activity from the get go when they arrived in america.

Another thing about gangs is that not every member of a gang that has 1000s of members is going equally involved in criminal activity or drug trafficking. Say if the gangster disciples have over 50,000 members the leader not going to be able to control all 50,000 its practically impossible. Thats why there are so many different sets.

heres some info i found on gang structures.
All gangs have identifiable levels of membership. These levels of membership indicate status within a gang and acts as the organizational maintenance systems. There are actually six levels of gang structure.

1. Leadership:
The leader(s) of a gang determines at what level of criminal activity the gang will function. Characteristics of the leader(s) are reflected in the day to day activities of the gang. The leader is all powerful.

2. Hard Core:
The hard core gang members are usually the older gang members, the individuals who are culturally and criminally enmeshed in the gang and are at risk of being so for life. Most violent gang activity emanates from the hard core gang members. Hard core gang members usually make up about 10% of gang membership.
3. Associate:
The associate gang member has usually made a personal commitment to the gang culture and is dedicated to achieving the level of recognition needed to attain hard core status.

4. Fringe:
The fringe gang member is still able to function outside of the gang structure and has not made a commitment to a life in the criminal gang culture. This type of member drifts in and out of the gang and seems to lack direction.

5. Wanna-Bes:
Wanna-bes are not actually gang members. They are youth who view the gang as an exciting place to be, a place where they could become "somebody". Wanna-bes may emulate gang dress, graffiti, hand signs, and other gang cultural symbols, and they may associate with known gang members, but they have not yet been excepted into the gang.

Cliques:
Very seldom is the gang at full strength. Exceptions to this, of course, would be times of conflict or possibly at social functions. What is most often seen as "the gang" is usually a clique from within the larger gang. The clique is a group of associate, fringe, and often, wanna-be gang members who gravitate around one or more of the hard core gang members. This somewhat resembles a gang within a gang.