There’s an important distinction between the crew (or NPC faction) and it’s gangs. Gang-size is not a count of the total size of a crew or faction. It’s the size of the gangs they can field.
For an NPC faction, the GM can decide to field as many gangs of that size as they think makes sense. Like the Bluecoats are Tier III, but there’s not a mere 20 Bluecoats in their faction. They’re a city-wide faction; they can easily field gangs of 20 anywhere in the city, as the GM decides and to the limit of what makes fictional sense.
It’s a similar idea for the player’s crew. The size of the crew is the number of PCs (maybe including lost and incarcerated), plus expert cohorts, plus each gang cohort (size per Tier). So a four PC crew, with one expert and three gangs, could count roughly 20 at Tier 1, about 40 at Tier 2, and over 60 at Tier 3.