For Bradley Cooper, it was less about needing to be mobile and more about needing to look like Chris Kyle, a former Navy SEAL and sniper who wrote the book American Sniper is based on. Chris Kyle was a large guy, bulky, who was later killed taking a vet who suffered from PTSD and schizophrenia to a shooting range.
Article source: https://www.cinemablend.com/news/2464425/bradley-coopers-trainer-never-thought-he-could-gain-the-weight-for-american-sniper