LAS VEGAS — Floyd Mayweather Jr. returned to the ring with another emphatic victory.

Maybe next time 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 return from retirement and a 21-month ring absence.

Mayweather knocked down Marquez in the second round and then peppered him with countless damaging shots to remain unbeaten (40-0, 25 KOs).

"I can get better," Mayweather said. "He's tough as nails."

Marquez (50-5-1) was a 130-pounder just 18 months ago, but the Mexican champion moved up two weight classes to be Mayweather's hand-picked comeback opponent. At Friday's weigh-in, he was four pounds lighter than Mayweather, who paid a $600,000 penalty for missing the bout weight of 144 pounds.

The size disparity was painfully obvious from the opening bell, but Marquez stayed on his feet for 12 one-sided rounds.

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 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 — and Mayweather clearly is still at the top of his game.

Mayweather was too heavy and too speedy for 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 end any IRS troubles while setting up another megafight.

Mayweather had a hefty advantage in this one, weighing in at 146 pounds Friday. He refused to re-weigh himself Saturday night, 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.

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.