If you are a tall or big guy, why do you ride short horses? It would make more sense to me if you rode a proportionally sized horse to your size, for a variety of reasons. One of which would be the proximity of your heels to the horse's belly.
Astride an appropriately sized horse, the rider's knees should have a slight bend and his heels should be close to the lower belly of the horse. I prefer short shanked spurs because I tend to over use the longer ones. Mine are straight and one inch long. Spurs are just an aid and should only be used if leg pressure doesn't get the desired results.
I don't know the technical philosophy behind it but I don't see an advantage to slanted spurs. Straight ones keep my toes out and heels down like they should be. Oh well, maybe an expert should have answered this one.
Just think, if it weren't for marriage, men would go through life thinking they had no faults at all.