Roy Jones, Jr.’s mouth dropped as Chris Byrd approached him last weekend in Las Vegas. Instead of looking like a guy headed to join Jones for a joint appearance on ESPN2’s Friday Night Fights, Byrd looked like he was walking into a Nutri-System commercial.

The former two-time heavyweight champion of the world was always accused of being too small to compete in boxing’s most important division, despite victories over giants Vitali Klitschko and Jameel McCline, as well as wins against Evander Holyfield and David Tua. Now he really is.

After being stopped in an IBF elimination fight by 28-year-old Russian heavyweight contender Alexander Povetkin last October, the 37-year-old Byrd realized he was at a career crossroads. The heavyweights had not only grown too big for him, they had also grown too young.

To continue to be competitive Byrd realized he would have to re-invent himself. So he decided he would move down to cruiserweight, figuring losing the 20 or so pounds necessary to reach the 200-pound limit would not be all that difficult. Well, a funny thing happened on his way to 200 pounds. He didn’t stop until he weighed 173.

“Once I stopped eating junk and started eating right the weight just flew off me,’’ Byrd joked last week. “When people said I wasn’t really a heavyweight they were right. I wasn’t a real heavyweight. I was a fat light heavyweight.’’

Fat or not, he was also a heavyweight champion twice over which is something a lot of fitter and bigger men will never be able to claim. But those heavyweight days are behind Chris Byrd now, and a new stage of an old career is about to open on May 16 when he fights at light heavyweight for the first time against Shaun George (16-2) on ESPN2’s Friday Night fights.

Byrd signed with promoter Art Pellulo after ending a long-running feud with Don King that had limited his ring appearances to barely one a year since 2003. With King no longer blocking him and his size no longer a problem, Byrd believes he is no more than a victory or two away from a shot at not only the light heavyweight title but a chance to become only the third former heavyweight champion in boxing history to move down to 175 and win another world title.

If he does it, Byrd would join Jones and Bob Fitzsimmons, who performed the feat more than 100 years ago, an elite short list that few heavyweights would even think of ever trying to join. But then, few heavyweights were ever built like Chris Byrd.

“I believe I can be competitive in the division and win another championship,’’ Byrd said. “I had no trouble losing the weight. It took me less than six months and it wasn’t like I was starving myself.

“I only eat one meal a day most days anyway and I’m pretty active. I just had to make it a good meal and not stop at McDonald’s on the way home from the gym. I feel great at this weight even though it wasn’t where I intended to be.’’

Where Byrd intended to be was competing with the world’s top cruiserweights, a division filled with big punchers like David Haye (who himself is moving up to heavyweight) and Jean-Marc Mormeck. Big punchers and small paydays is not the combination anyone is looking for if they can avoid it and once Byrd began to change his diet and increase his workout regimen he suddenly found he might be able to.

“The cruiserweights are still big guys,’’ Byrd said as he eyed an incredulous Jones staring at him his mouth agape from across the room at the Planet Hollywood casino. “I turned pro as a super middleweight 15 years ago and when I moved up to heavyweight nobody thought I had any chance to be successful.

“I don’t imagine many people thought I had a chance of moving back down to light heavyweight either but here I am. I expect to be dominant at this weight.’’

The stylish southpaw has never been a big puncher, as his 21 knockouts in 45 fights attest. His has always been a game based on slick defensive moves, ring intelligence, quick hands and the ability to do his business and then move out of harm’s way before big galoots like McCline or David Tua could return fire.

Yet in later years, as he began to slow down as all fighters do, Byrd found himself more and more often forced to stand flat-footed and fire away against towering men like Wladimir Klitschko, who twice stopped him, or get up off the deck against McCline and find a way to win against tall odds.

But the loss to Povetkin was the final straw, a defeat that convinced him it was time to seek a new job or a new way to stay alive in his present one. Much to his surprise he found the answer in a division he never expected to compete in, but one where he believes he will finally have physical advantages as well as skillful ones.

“If I can’t hurt these guys then it’s time to stop,’’ Byrd (40-4-1, 21 KO) said. “I really think I’ll be bigger and stronger than these guys. And the top light heavyweights are the same age as I am. I really believe this will be a new chapter for me.’’

It’s a chapter that will have to include at least one or more guys like a trio of 39-year-olds, Jones, Glen Johnson and Antonio Tarver, as well as 36-year-old Joe Calzaghe, who just eked out a split decision from 43-year-old Bernard Hopkins last week. What it likely would not include is 25-year-old WBC champion Chad Dawson. With so many of his contemporaries around, why mess with someone who wasn’t old enough to box when he began his professional career?

Jones, Tarver, Johnson and Calzaghe are Byrd’s true contemporaries but more importantly they are also his physical peers. To compete against the latter will take Byrd back almost 16 years, back to a time when he was winning the silver medal at the Olympics in Barcelona as a middleweight before turning pro and moving up from 168 to heavyweight after just three professional fights.

At the time Byrd’s decision was considered foolhardy by most boxing insiders but he has had the last laugh on all those who said a little man without a knockout punch could not long survive among boxing’s redwoods.

It has been a long road from that final amateur tournament to this new beginning at 175 pounds but this last chapter will not be like his 14 years in the heavyweight division. This will be a short story regardless of how quickly he gets his shot at joining Jones and Fitzsimmons in boxing’s history books.

Byrd’s hope is clear. He would like to see Jones dethrone Calzaghe next fall in England or Wales to win the linear light heavyweight title and then get his own chance to face the four-time world champion (middleweight, super middleweight, light heavyweight and heavyweight) in a big-money mega-fight that would be more about speed, skill and elusiveness than ponderous power punching.

“I’d love to fight Roy,’’ Byrd said. “I don’t know what he wants to do but it would be a great fight. Two former heavyweight champions fighting for a light heavyweight title! That would be history.

“A lot of guys go up in weight and some of them find ways to keep winning. Not many guys go up to heavyweight and then go back in the other direction. Once you gain that weight it’s too hard to lose it, mentally as much as physically. You just don’t want to do what you have to do but that wasn’t a problem for me. Keeping the weight on was always my problem.’’

Not any more. After 15 years and two reigns as heavyweight champion Chris Byrd is finally where people always said he should be. He’s in a division where he can finally pick on guys his own size.