Is my Dachshund Overweight?

Ive had many people tell me that my Miniature Dachshund is overweight. She weighs about 15 pounds, she is EXTREMELY active she is constantly in the backyard running around or playing with her toys or my other dogs. When i look at her from above she doesn't have a curve to her waist, its more of a straight line, shes a year old, shes spaid (spelling?). she eats only dog food except for the occasional gummy bear. The only thing i can think of that might make her overweight is that since i have 5 dogs (Great Dane,Chihuahua, Chihuahua pomeranian mix, and ****-zu wired haired terrier mix, and dachshund) and they are all over 7 years except the dachshund so they all free feed. She doesn't sit at the bowl all day long and eat its more like she grabs a few nibbles throughout the day and eats at night. Can someone tell me what i can do to bring her weight down. please don't tell me to stop the free feeding because the other dogs are not overweight and they do the same thing she does. Thanks :)