So, the military is legally allowed to discriminate in certain categories– anything having to do with physical ability, essentially. Certain physical fitness standards must be met and kept and there are regular tests done to measure this: a certain number of push-ups, sit-ups, and a 2-mile run for the Army (pull-ups are done by the Marines as well). How many you are expected to do is based on a sliding scale that takes your age into account; there’s also an allowed body fat content that depends on age, weight, and height.
Despite that, it’s possible to serve while overweight. Most of the time the chain of command will get on a soldier’s case about it, but some are remarkably lenient about who they overlook when enforcing these rules. In the old days, a unit’s chain of command was supposed to give a Physical Fitness, or PT, test to make sure the students they sent to development courses were fit within Army standards. But people played favorites and sent who they wanted, including people who violated the standards.
Now, most development academies administer their own PT tests when the students show up. It’s an extra hoop everyone has to jump through because a few people tried to game the system.
The funny thing is, despite all the focus on physical fitness, it is always amusing to see Hollywood movies with actors portraying soldiers in ideal physical peak, with bulging muscles, rock-hard abs, and 2% body fat. In the comic, Rock and Dawg are pretty normal examples for their respective ages.