My Breeder who also shows maintains all are individuals, They should look right but have a waist line. Mine weighs 11kilos at 8.5 months and is fit and healthy.We in England let them carry more weight then America.
My Mini Schnauzer is 7 months old and weighs 18 lbs. I just follow the reccomended food amount on the bag, and one short walk a day(If we can get that in). playing around the house is usually sufficiant, and he is healthy.
You might try changing his food. For example, my mini was eating Science Diet and put on quite a few pounds. We switched to Iams and he shed the extra weight and was happy as could be. I don't know a lot about dog food ingredients, but it would be worth checking into for the overall health of your puppy.