I said relatively small pond, Olivant. I meant "small" relative to what Michael controlled and strived for. Sure, Vito was the top Mafia Don in New York in his era--and New York had the richest rackets. But other than his influence on the Hollywood unions, his writ didn't extend beyond New York City, where he was the biggest--but not the only--Don. Michael arranged for the murder of the other NYC Dons, leaving him in a far more powerful position than Vito held in New York. Michael extended the family's influence to Nevada by arranging for Moe Green's murder, then owned or controlled three Nevada casinos and was acquiring Klingman's interest in another. Michael was the heir-apparent to Roth's Cuban gaming empire. And he still controlled New York through Pentangeli. At the Senate hearing we learn that he "controls all the gambling in America." Even in GFIII, when he's supposedly totally legit, we learn from Vincent that he still controls the Commission and is taking over control of International Immobiliari.
Vito knew his limits and kept his appetite for power under control. Michael didn't. Vito was relatively more successful in what he tried to achieve, while Michael constantly won battles and lost wars.