To further the comparison, not only are some men's penii (plural) larger, you can't exercise a penis to make it bigger (permanently, that is :-)), the same goes for breasts. Breasts and penii can be surgically enlarged.
1993, Rita Mae Brown, Venus Envy, Random House, pp. 239-241:
He delicately poured white lines of cocaine on these erect penii which the white members of the party and crew sniffed.
1994, Fay Ajzenberg-Selove, A Matter of Choices: Memoirs of a Female Physicist, Rutgers University Press, p. 95:
Its [Boston University] campus was filled by a complex of buildings dominated by two towers called by its inhabitants "the twin penii".
1997 [2004 reprint], Matt Ruff & Tom Stoppard, Sewer, Gas and Electric: The Public Works Trilogy, Grove Press, p. 67:
[T]he United States Supreme Court had determined […] that artificial testicles were kosher under this law, but prosthetic penii clearly were not.
2004, Howard Korder, A Sea of Tranquility [play], Dramatists Play Service, p. 50:
RANDY: […] The penis cannot be bottled ¶ ASHLEY: Okay, I see, well — ¶ RANDY: No bottled penii. Hilarious!
2004, David Bret, Morrissey: Scandal and Passion, Robson, p. 132:
[I]n August 1993 he [Richard Smith] wrote a feature for the Gay Times entitled 'Cock Rock', which detailed 23 of 'the most noted penii in pop'.
2005, Kevin Dole 2, Tangerinephant, Afterbirth Books, p. 27:
The audience as canis: hyenas and jackals, tongue lolling, penii flopping, end over head over end.
2006, May 14, Neva Chonin, "Floppy Joe", San Francisco Chronicle, p. PK-18:
Hey, I'm talking about penii again, an appropriate topic for the month marking the 150th anniversary of Freud's birth.
2007 May 6, Stefan Gates, "Taboo foods", Cooking in the Danger Zone, BBC:
More dog willies. What’s with the penis? Why so many penii?
2008, Chuck Austen, Like Warm Sun on Nekkid Bottoms, Lulu.com, pp. 289-290:
Interestingly, Nightcrawler had two penises.//Penii?