You need to buy a premium dog food for weight control such as Iams or Science Diet W/C and this will help to slim her down some. Also since she has been spayed this can cause weight gain as well so you need to promote frequent exercise as much as you can and limit her meals to two a day and not leave food out all the time for her to eat on. She needs to be on a regular feeding schedule and these things will help to start getting her weight under control. Don't expect changes overnight but over the next few weeks you will start to see a change in her weight as long as you use a weight control formula dog food. Even Purina has an indoor and weight control formula dog food so one of these would be your best bet.