It's probably a combination of hand size and strength. Can't imagine big hands don't help hang on to the ball. Obviously keeping the ball next to your body is a must so there is more to it than just hand size.
I recall that one of Ahman Green's problems was that he was lefthanded, and - unlike most RBs - he never learned to feel comfortable carrying the pill in his offhand.
So when he ran to the right, the ball was exposed and vulnerable to linebackers and safeties spearing it at full speed or stripping it. Most of his fumbles came when he was running to the right, and other teams definiteluy figured it out. Brian Urlacher sure took advantage of it; he said it straight up after a game where Chicago embarrased Green Bay in 2004, partly because he stripped Green of the ball deep in the red zone.
Green's left-arm reliance is hardly a secret. Just ask Bears linebacker Brian Urlacher, who darted through the offensive line and cleanly stripped the ball from Green.
"He carries the ball in his left hand and I just happened to hit the ball," Urlacher said."
They say you can't teach an old dog new tricks. One flaw in that philosophy is Ahman Green is neither old nor a dog.
247sports.com
Green knew it, too, and I recall the coaching staff worked with him on it. But he never was able to correct it, even once he knew how much a part of the problem that was. He just couldn't learn how to feel natural carrying a football in his right hand. So, 4-7 fumbles every season.
But - once he went to Houston, he never put the ball on the ground again in his life. He did have fewer than 200 more touches in his career after going to Houston, but still, considering he averaged 1 fumble every ~50 carries through his career, it's statistically significant that he never dropped another ball in his last 3 seasons. That suggests he may have finally figured it out.
So yeah, sometimes really simple things play a big part in the problem. That's part of why I say maybe Green Bay's scouts and coaching staff have seen something in Lloyd's game that they feel is correctable. I reckon we will find out.