Lifting weights at the gym and eating a heavy meat diet high in protein will help build body-mass and increase muscular strength. Muscles are heavier than body fats, and while carbohydrates provide the energy required to lift weights at the gym, any energy left unused at the end of the weight-lifting session will turn into body fat. It is important to eat a high-protein meal after a weigh-training session to provide the muscles with nutrients required to restore the stressed muscles and increase volume and strength.
Yoga works with the body’s innate resistances against bone and antagonistic muscles. Yoga works with the body’s natural ability to stand, sit and operate on this planet under the effects of gravity. Yoga does not build large muscles and does not lead to gaining body-mass larger than that required to operate optimally on Planet Earth. Yoga builds, develops and strengthens muscles at the core of each muscle group.
Picture yourself in Virabhadrasana II (Warrior 2). With pelvis open toward the side, the torso is straight, and the arms are held out horizontally, supported by the lateral muscles and the shoulders. The strain that one feels in this pose derives from muscles being worked deep inside the arms. Muscles right near the bone. These muscles, when engaged and strengthened, keep the body protected in situations when we need to apply greater resistance to the forces of gravity than is natural.
Such an example may be lifting weights in a gym.
So while lifting weights may deliver the results you initially consider to be most important, a regular Yoga practice will make your weight-lifting a safer practice, and more sustainable.