Why San Francisco 49ers Star Nick Bosa’s Thighs Are Noticeably Different Sizes: ‘Atrophy is Real’

Size doesn’t matter to Nick Bosa.

We’re talking, of course, about his thighs. The San Francisco 49ers defensive end appeared at this year’s camp sporting one thigh that was noticeably larger than the other.

“Atrophy is real,” Bosa, 28, told reporters on Tuesday, September 8, revealing that an October 2025 ACL surgery and the subsequent rest and recovery time made his right thigh considerably weaker than his left.

“I don’t remember [the size difference], but it was drastic after surgery,” he continued, per The Athletic. “It’s going to take a couple of years to get that last couple of centimeters.”

Bosa played just three games in 2025 before suffering his season-ending injury in a game against the Arizona Cardinals. Eleven months later, he’s not in a rush to rebuild his right leg

“It’s not really a focus for me,” he said. “I’m not going to change my training just to get size. I don’t train like a bodybuilder — I train like an athlete. I’m not going to just add exercises that aren’t good for being an athlete to add size onto my leg.”

He added, “My leg is operating just as good as the other one. That’s all I really care about.”

Despite Bosa going nearly a year without appearing in an NFL game, 49ers general manager John Lynch isn’t concerned about him being ready for Week 1.

“I always believe that the athlete knows their body better than anyone, and Nick certainly takes that to the nth degree,” he said. “He knows his body extremely well and knows what it takes to have him ready. We saw him one year in a contract year come in here like on a Thursday and play pretty well on Sunday. So we’ve got a lot of faith in his ability to do that.”

It also helps that Bosa has been down this road before. He tore his ACL in September 2020 and was still on the field to start the next season thanks to his rehab and a rigorous diet. Looking back at the experience, Bosa admitted his habits might not be quite as disciplined this time around.

“I don’t know if I’ll be that strict again,” he said. “It was just a little over-the-top in terms of cutting out carbs and stuff.”

One similarity to his last ACL recovery, however, is that he feels faster than before.

“I do feel some similarities to that year, for sure — a little slimmer, lower-body-wise,” he said. “And I’m moving really good. So we’ll see if that manifests itself on the field.”