Derek Brunson shows incredible muscle gain in first PFL shots at 39 compared to his UFC days... but MMA fans think there's a simple explanation behind his striking body transformation

Derek Brunson said he was open to fight in any weight class after joining the Professional Fighters League (PFL) last month and looks to have kept his word.

If his first official portrait for the PFL is anything to go by, the 39-year-old is in the shape of his life and looks even leaner and bigger than during his time in the UFC.

In his UFC official portrait, Brunson looks as muscular as one may expect of a professional fighter, with his 6ft1in, 13.3 stone looking imposing. 

But the 32-fight veteran's muscles look even bigger in his first snap as a PFL fighter with his abdomen and pecs looking more defined. 

The two shots of Brunson were juxtaposed on X, the platform previously known as Twitter, by the Andy Hickey MMA account.

The post noted the difference between the two pictures was 'striking' and particularly impressive given Brunson is six weeks short of his 40th birthday. 

But MMA fans believe the difference in physique was nothing more than an optical illusion due to Brunson being lathered in what appears to be oil.

Others, meanwhile, suggested The One looks far bigger in his PFL portrait because the picture was taken with better lightning and with a grey background as opposed to a white one.   

'The UFC pic combo of rim lighting and a white background is actually making him look smaller. If the PFL pic the rim light reads correct,' one wrote.

Another added: 'Looks like they just lathered him in baby oil to me. Muscles look the same, just oil to make him look more defined and different lighting.' 

A third, meanwhile, noted: 'They look basically the same lol. Sure he’s not the best fighter ever but the man is definitely an elite athlete. This isn’t uncommon for people like him.'

Brunson has historically fought at 185 pounds during his 13-year career, but the PFL doesn't currently have a middleweight division.

The American will face two-welterweight champion Ray Cooper III on Friday and insisted he was happy to compete in any weight division.

'I can make middleweight, I can make light heavyweight,' he told MMA Mania earlier this week.  

'I can make heavyweight, so if they need somebody to fight Francis Ngannou, you never know.' 

Brunson is one of a string of high-profile free agents along with Francis Ngannou and Jake Paul who have signed for PFL over the past 12 months.

And PFL chairman Donn Davis believes the promotion's rosters will get even better after completing its merger with rival Bellator earlier this week. 

'Our cards are gonna be better [next year],' he said. 

'It’s just a matter of time before that starts to get more well-known and we’re not number two [behind UFC], but we’re a co-leader.'