Promoter Bob Arum says the size differential between welterweight beltholder Manny Pacquiao and middleweight champ Sergio Martinez is too great to make a fight between the two superstars, currently ranked Nos. 1 and 2, respectively, in THE RING magazine's pound-for-pound ratings.

Pacquiao (53-3-2, 38 knockouts) began his professional career at 106 pounds in 1995, and the heaviest he's weighed in is 145 3/4 pounds — his weight for the Joshua Clottey bout in March of 2010 — and he doesn't add significant weight between the weigh-in and fight time. He stands 5 foot, 5 1/2 inches tall and has a reach of 67 inches.

Martinez (47-2-2, 26 KOs) stands 5-10, has a 75-inch reach, and made his pro debut at 146 3/4 pounds. He weighed in at 159 1/2 pounds for his 2009 victory over Kelly Pavlik, and was 158 3/4 in his most-recent bout, a TKO of Serhiy Dzinziruk in March.

If Pacquiao fought Martinez, it most likely would be at a catch-weight of 150, Arum said, but he doesn't think Martinez could reach that weight.

"It's a fight I believe Manny could win, but it's a fight that's a little crazy because Manny gets in the ring (on fight night) at 147 or 148," Arum told Chris LaBate of BoxingScene. "Sergio Martinez, even if he could make 150 pounds — which I don't know if he can — gets in the ring at 175. You're asking Manny to spot a fighter like Sergio Martinez 27 pounds, which is unheard of."

Meanwhile, Arum didn't entirely rule out the possibility of a Pacquiao-Martinez collision at some point.

"I think Manny can win the fight, but it would take a lot out of him," the promoter said. "And maybe down the road we'll do that, but I wouldn't recommend it."