When it comes to overweight, it is pretty much the same causes for both dogs and humans. If they eat more calories than they spend, the extra energy will be stored as fat. It is that simple. So either decrease the calories intake, or increase the spending, or both.
Cut back on feeding should be the first thing to do. The total food you give to them on daily basis should be reduced. You can also give the dogs smaller more frequent meals instead of just one or two large meals. Maybe improve food quality is also needed. Higher quality food has less filler and other garbage.
Then there comes activity level. Walk your dogs more often, play with them more often should do the trick. It is also good for your own health if you walk and play with your dogs often.
