Actually, the smaller dogs seem a bit less prone to injuries.   And, it is a lot easier to haul a 40 pound dog than a 72 pound dog in a sled, if necessary.  I do think the Quest overall has fewer dog fatalities in part because the dogs and the mushers have a mandatory 36 hour rest.   If you've seen the mushers at checkpoints or in the Insider videos, you cannot help but be struck by how exhausted they look.  It's hard to make sound judgments for yourself OR for your dogs if you are sleep-deprived for days, in my view.  I think most mushers generally try to avoid dogs which are "inbred" in order to avoid problems, although accidental breedings do happen.   I sometimes neuter all the males in a litter or else spay all the females in the litter to avoid that problem, although I must confess I had an inbred litter from an accidental breeding back in 2012 and both dogs finished the Iditarod in 2014; one is an excellent leader and both are still racing.