All dog breeds can become obese. Some seem to be more prone to it, either because we've bred them to be incredibly food motivated so they will overeat excessively if allowed (labs are a classic example), others because their anatomy makes exercising more difficult (English bulldog, pug), and still others because they're prone to metabolic or thyroid disorders that cause obesity as a side effect (Cushing's in mini poodles, for example).
But any dog can be overfed or fed a poor quality diet and become obese. Some individuals will self-regulate and only eat what they need, or even be picky eaters or 'hard keepers' with high metabolisms and need extra help to keep weight on, but most will tend towards gaining weight if their food and treat consumption isn't tailored to their needs.
While some breeds might be more prone to being overweight/obese, barring an underlying medical condition, it's ultimately the owner's fault if they wind up that way. Knowing what a dog at a healthy weight looks like, and then keeping an eye on the dog's condition, and adjusting their food and exercise appropriately is up to the owner. If you notice weight gain without a change in diet or exercise, or the dog can't lose weight despite reducing the amount of calories they take in and/or increasing the amount of calorie they burn, then a vet visit to see if there is something like hypothyroidism going on.
Over the years my observation of all different size dogs has led me to the conclusion that it is an owner problem. I have noticed more smaller dogs than larger dogs overweight. I think that may be because there is a tendency for larger dogs to get more exercise from their owners but it is only an opinion. I used to think that owners that left out the dog food all day were a big reason for a fat dog but have come to the conclusion it is more the feeding of table scraps to a begging dog. Who could resist those eyes? It is pretty easy to keep track of your dogs weight. Decide the optimal weight and weigh the dog occasionally or feel in the rib area where you should just be able to feel the ribs. Most owners of fat dogs seem to just shrug it off. A friend at the Univ. Pa. Vet Center once told me that an overweight dog can cut about 10% off a dogs life.