Your bones lose density from lack of use. Calories turn to fat when you don't burn them off with activity. The heavier you get, the harder it is to exercise and so to lose weight. Extra weight adds to the strain on your heart and muscles. You don't have to be a bodybuilder to maintain strength.
Muscle strength is as much a matter of softness and elasticity as it is of bulk.
Flexible muscles can travel further when they contract, which translates to additional strength. Whatever muscles you have voluntary control of can be exercised and kept at an optimal level of mass and elasticity. You can do this without an extreme amount of exertion. It is a myth that to exercise a muscle you have to strain it to exhaustion. Gentle and slow repetitions with little weight will maintain muscle tone, at least, and gradually build strength, at best.