I am 6 feet tall, have a stocky build, and am 41 years old. I have always been athletic and lifted weights seriously for 15 years between the ages of 20 to 35. I let myself go for a few years and then got myself into shape and got down to 195 pounds. I stayed between 195 and 200 for a few years. I started to creep up last summer and was around 203. I thought I might be gaining lean muscle from the work outs I do on the elliptical machine. I then started working out with weights a couple of days a week to go along with the elliptical workouts. I train really hard and am in great shape. I am at 206 though now and am wondering if I am gaining lean muscle. My clothes fit the same, same pants size, but I have filled out in my upper body from working out. My question is, am I gaining lean muscle or fat, what the heck is going on? I haven't changed my diet and I work out almost everyday either using the elliptical machine or weights. I feel great and am in super shape. I usually burn 1200 calories on the elliptical and go for at least an hour.
If you gain weight but your waist is still the same size then the gain is muscle not fat.
From hearing about how tall you are, I believe it's a good weight and you are gaining muscle. Also you said that your waist is staying the same so I believe it's mostly muscle. The first place to monitor to see if you are gaining fat is the waistline because that will be the first place fat will show up. If you keep doing the elliptical and lifting weights, you'll get the results you want so keep it up.