He looks big but don't really look very overweight but legs are quite thin so I guess you're right. Then, a bit curious, I went and search and found this:

As might be predicted, given their large size, Mastiffs suffer from a number of joint and structural problems. It's important that young, growing Mastiffs be kept lean and not allowed to exercise too strenuously or eat too much, as this will lead to injuries and problems that can be crippling down the road. In fact, all Mastiffs need to be kept lean, as obesity increases the chances they'll develop certain structural problems and makes them more painful when they occur.