In areas where rust is a problem the bigger armor can hold mud and salt behind it and cause the housing to rust and leak.
CAN?? You can safely change that to WILL hold mud, sand, etc.
Isn't the *small* one made of out of the same thickness as the *big* one? So as long as you hit a rock on the protected part, you're OK. But don't hit a rock hard on the unprotected part as you'll put a dent into the housing, and if hard enough, out comes the oil.
I wear a full cover (pipe cap) and attempt to keep the sand/dirt out of it. I don't worry about hitting rocks anymore.