good question mate and one ive wondered myself ,I have an 8 month old bull cross who was very stocky and probably a little overweight i increased his exercise to about 2 hrs maybe a little more of just walking some ball throwing etc i kept the amount of food the same though and hes started to tone up though he still too young with soft bones to rreally get him fit,when hes old enough will get him into condition by working him. i think the thing to do is increase exercise before you cut down his food but im only learning myself atb matt tyke in the top pic 4 1/2 mth and 6mth in bottom