When someone enlists into the Marines, they have physical qualifications, including weight to height ratio. Before my retirement, enlistment papers would come across my desk. Normally, I only checked numbers to see if our enlistment centers were making their numbers. Besides our enlistees being rejected for having issues unknown to them, like Diabetes, Heart Disease, etc., Obesity was usually among the top 3 reasons why candidates were rejected. However, if they are within range of being in the standard category, they may be given a waiver and allowed to stay, if they get their weight in the standard category by the time basic training is over.
If someone is in excellent condition, but later allows themself to gain too much weight and is not acceptable to the standards, that Marine will be “volunteered” to join a program to get the person’s weight back under control. You should never see a fat NCO.