J-Doug of the excellent Rational Pastime blog is now writing for Beyond the Boxscore, and he has an excellent post up on relievers who get a wide strike zone. Mariano Rivera benefits from one of the largest zones, as this graphic shows:
[image title="rivera_medium" size="full" id="22929" align="center" linkto="full" ]
Here is the typical strikezone:
[image title="lateinnings_medium" size="full" id="22930" align="center" linkto="full" ]
I recommend reading the full post, as it includes a number of interesting tidbits on Rivera and discusses various other pitchers who benefited from a large strike zone. Regarding Rivera, he had the third largest advantage from a large strikezone among pitchers who threw at least 500 pitches since 2009. I think most Yankees fans have always recognized that Mo benefits from an expanded zone, likely because he is always near the plate with a cutter that may deceive umpires much as it fools hitters. A pitch with late break is among the most difficult for an umpire to call, and being that it is the only pitch Mariano throws, he would benefit from this bias constantly. J-Doug has some other theories, and again, I highly recommend heading to the site and checking it out.