I agree totally with previous poster, my vets have always, for as long as I have had giant breeds, about 25 years or so, said that of course be careful with how much you work them, especially when growing, but one of the biggest risks is to get a giant dog fat...... Okay lol..... chunky sounds better, the heavier the more strain on joints and bones, so rather than having a too heavy dog, it is better to go for the little leaner look. Mind you if you conformation show, they want them kinda wide, not too, but definately not on the too lean side.
It must have been a sight lol..... your big guy doing hurdles. Are you still doing agility with him??
Marit