At rest, one pound of fat burns about two calories per day, while one pound of muscle burns about six calories. Simply having more muscle is not going to make that much of a difference all by itself. Building five pounds of extra muscle is only going to add about 30 calories per day to your resting metabolic rate, but while you are cutting calories to lose weight, it's very difficult (if not impossible) to add five pounds of muscle to your body.
Muscle does burn more calories at rest and at work. But losing weight always involves losing some muscle along the way. If you don't strength train, as much as 30% of the weight you lose could be muscle weight, which works out to about 15 pounds of muscle loss for your 50-pound weight loss. Losing 15 pounds muscle could cost an active person several hundred calories per day in lost calorie burning capacity since active muscles (as opposed to resting) burn 30 times more calories than fat. This is one of the big reasons why people who lose weight rapidly on crash diets (without exercise) almost always put that weight back on later. However, an effective strength training program can reduce that 30% muscle loss figure to 3-5%. That's much better for your fat burning goals and your metabolism.
Fat-burning tip: Even if you're short on time, you shouldn't give up the cardio and concentrate on strength training if you want to burn fat. You'll burn more fat by doing 30 minutes of cardio than you will in a whole day sitting around with five extra pounds of muscle. But don’t make the opposite mistake of thinking you don’t need strength training, either. Both are very important for fat burning and overall health.
When you put all this information together, the picture you see is very familiar. There aren’t any gimmicks or shortcuts. To burn fat, the variables are simple. You need to: