Money came back with a guarantee. Floyd Mayweather Jr. told the world he was still its best boxer despite taking 21 months off, and he backed up every boast with every jab in an impressive victory.

Although next time out, hopefully he'll pick on somebody his own size.

Mayweather overpowered the smaller, lighter Juan Manuel Marquez for an unanimous decision Saturday night, maintaining his perfect record in his comeback from retirement.

Mayweather knocked down Marquez in the second round and then peppered him with countless damaging shots to remain unbeaten (40-0, 25 KOs). Still, his impressive effort couldn't be viewed without weighing the obvious disadvantages in height and strength faced by the talented Marquez, a 130-pounder just 18 months ago who moved up two weight classes for this once-in-a-career payday at the MGM Grand Garden.

"Marquez is tough as nails," Mayweather said. "He's a great little man. He was really hard to fight, and he kept taking some unbelievable shots."

At Friday's weigh-in, Marquez (50-5-1) was four pounds lighter than Mayweather, who paid a $600,000 penalty for missing the bout weight of 144 pounds. Simple physics took over from there. Though Marquez stayed on his feet for 12 one-sided rounds, Money Mayweather's win was never in doubt.

"He's a great small man," Mayweather said. "Don't forget, I came from a small weight class too, so I know when you're in front of a great fighter. I think he brought his best tonight."

Mayweather had an astonishing edge in punch stats, landing 290 of his 493 blows (59 percent) while allowing just 12 percent of Marquez's 583 punches to land. Mayweather landed more jabs in each round than Marquez landed total punches, and just 16 percent of Marquez's power shots even got to Mayweather.

Mayweather often appeared to be toying with Marquez, who's generally considered among the world's top handful of fighters. Just 18 months ago, Marquez lost a narrow decision to unofficial pound-for-pound champion Manny Pacquiao _ another mighty mite who's likely Mayweather's top choice for his next bout.

Pacquiao accepted a similarly mismatched challenge last year when he demolished Oscar De La Hoya, but the Golden Boy acknowledges his skills have diminished. Mayweather clearly is still at the top of his game.

"I've been off for two years, so I felt like it took me a couple of rounds to really know I was back in the ring again," Mayweather said. "I know I'll get better."

Mayweather dominated his undersized Mexican opponent in his first fight since stopping Ricky Hatton in December 2007. He then took a lengthy break from the sport that's dominated his life since he was a toddler, but returned for another eight-figure payday that should assuage the IRS while setting up another megafight.

Mayweather refused to re-weigh himself Saturday night, so the true size of his advantage might never be known. Fighters often gain several pounds between the weigh-in and their bouts.

But his size advantage was obvious from the opening bell, when it became clear Marquez would struggle just to get close enough to throw good combinations.

"He surprised me with the first knockdown," Marquez said. "He hurt me in that round, but not any other time. I don't want to make any excuses, but the weight was the problem. He's too fast."

Judge Burt Clements gave every round to Mayweather, 120-107. Dave Moretti threw the eighth round to Marquez for a 119-108 total, while William Lerch gave two rounds to Marquez, 118-109. The Associated Press had a whitewash, 120-107.

Mayweather abruptly knocked down Marquez midway through the second round when Marquez walked into a left hook, but Mayweather largely stuck to his jab, leaned back in his familiar defensive posture and picked apart another opponent.

Marquez had a bloody nose by the bout's midway point, and Mayweather landed several hard shots late in the sixth. Whenever Marquez appeared to land a combination, Mayweather invariably backed away with a grin.

"When I hit him, he laughed, but I knew he felt my punches," Marquez said. "We tried to work the speed, but the difference was the weight."

Marquez added a brave defeat to a career noted for its disappointments as much as its triumphs. Marquez still seethes over his 0-1-1 record in two fights against Pacquiao, and he followed Pacquiao's path up in weight in search of bigger bouts, which he got in recent wins over Joel Casamayor and Juan Diaz.

Mayweather chose Marquez for his return bout after flirting with Pacquiao, who will fight Miguel Cotto in the same arena less than two months from now.

A Mayweather-Pacquiao fight would be the biggest in the sport, but a more natural opponent also wants a little Money. Sugar Shane Mosley called out Mayweather in the ring immediately after the fight, with Golden Boy representatives forced to separate the two.

The lukewarm public response to the bout underlined industry concerns Mayweather can't sell a pay-per-view on his own. The Grand Garden wasn't sold out just a few days before the fight, and large swaths of empty seats greeted the undercard fighters, though they were mostly filled before Mayweather entered the ring.