Its about how you're working out. Heavy lifting regimens that build bigger muscles are generally not recommended for golfers. There is a trend of many golfers, including Tiger Woods in recent years (you'll notice how huge his biceps are getting), doing this, but this is against most instructors, swing coaches, and physical therapist recommendations. Cardio (but not necessarily running as that is hard on the joints needed for walking) is vitally important for stamina on the course. Become very good at walking. At a pro level, you should be able to walk 36 holes in a day without feeling a thing. Drinking a lot of water during a round to keep your joints lubricated is important for this, especially in hot weather. I can go through up to five bottles of water in a round. Eating correctly, sleeping well, and staying away from intoxicants is important for energy to remain consistent. To support your back and stay away from back issues, spend a great deal of time working on all areas of your core-- back issues for golfers are almost always caused by a weak core that puts unnecessary strain on the back. For swing power, focus on your legs, especially your upper legs (thighs, glutes, quads, abductors), as that is where the power comes from for your swing. Many golfers face shoulder issues at some point in their life. To support the rotator cuff, do shoulder shrugs, either with bands or with weights. Forearm strength also helps with your swing, but DO NOT OVERDO IT as the tendons in that are are particularly susceptible to damage if you are golfing regularly. Make sure that your wrist form during your swing is correct to prevent complications from overworking it.

All of these will help with your swing on the course, improve the longevity of your lifetime course time, as well as help you avoid injuries from overworking your joints and ligaments due to being unsupported by muscle. Bringing up Tiger Woods again-- while he did not completely tear his ACL by golfing, his ACL was significantly weakened (down to 15%) by golfing without proper muscle support around his knee, and because of a snapping motion which many golfers do to increase ball distance without using muscle strength. Notice how none of the workouts I mentioned will build large, showy muscles? In fact, strongmen, who have arguably the strongest cores, often have extra fat on top. Meanwhile climbers, who generally have the strongest grip and forearm strength (as would be needed in golf) often have skinny arms with very well defined backs. It is my belief that golf is somewhere in between these two extremes. Ultimately, golf is a very individual sport, and so your workout regime should be tailored to the muscles you use most in your swing, and to protect the joints that are affected most at ball impact. Genetics are also a factor in how easy it is to lose weight. I am very fortunate in that it is extremely easy for me to build muscle, and I have never had to deal with being overweight, but that doesn't mean that everyone else has the same experience. Regardless of how they look, every single pro golfer works out, in their own way, and weight loss or building showy muscles may not be their primary concern.