The Corleone Family chart, and the Senate hearing scenes, were FFC's perfect clone of the Valachi hearings, right down to the design of the charts and the hearing room. Go here for the original:

http://www.gangrule.com/galleries/family-charts

The inaccuracies of the Corleone chart also were a reflection of the charts in the Valachi hearings. The FBI hadn't paid much attention to organized crime until Valachi made them sit up and take notice. Then they scrambled to put together their charts to make it seem as if they knew all about the Mob. Valachi was the source of much of the info. But writers have pointed out that Valachi was too low in the organization to be definitive. So, if Cicci was the movie committee's Valachi, he might have known some of the higher-ups, but not everyone. Hence relatively small size of the Corleone family as depicted in that chart.

On the other hand, Michael would not have needed a large organization in Nevada, especially since he was trying to maintain a "legitimate" front. Most of his muscle would be in New York. And, since it'd been only a few years since Michael arranged to wipe out the other NY Dons, perhaps Clemenza, and then Frankie, didn't need as large an organization as before.